RSS feed for Teaching mathematics

Skip transcript: Video 2 Dividing 136 by 4 using partitioning

Video _unit2.2.1 Video 1 Using the empty number line to subtract 238 from 402

Interactive feature not available in single page view (see it in standard view).

We can use this method to answer questions such as £4.02 – £2.38 or 4.03 m – 2.38 m, taking us into the realm of using measures. This method also helps with time calculations where learners need to remember that time is not a decimal system. For example, a train journey which starts at 2.38 pm and goes on until 4.02 pm takes 22 minutes + 1 hour + 2 minutes which comes to 1 hour 24 minutes (bridging through 3 pm and 4 pm).

Activity _unit2.2.2 Activity 4 Subtraction problems for you to solve

Allow 5 minutes

Try answering the following in your head using whichever method you prefer.

You may check your answers using a calculator.

2.3 Mental multiplication and division strategies

The Open University — Wed, 01 May 2019 17:18:55 GMT

Doubling and halving are easy for most people, so make use of these tactics:

doubling and doubling again is the same as multiplying by 4
doubling and doubling and doubling again is the same as multiplying by 8.

Use the inverse of doubling (i.e. the opposite of doubling) which is halving:

halving is the same as dividing by 2
halving and halving again is the same as dividing by 4
halving and halving and halving again is the same as dividing by 8.

Once we begin talking about halving being the same as dividing by 2 this connects to fractions, decimals and percentages. There will be more of this in Week 2 but for now:

half of 100% is 50% → to find 50% of an amount just half it
half of 50% is 25% → to find 25% of an amount half and half it again.

Activity _unit2.2.3 Activity 5 Finding halves and quarters

Allow 5 minutes

Try the following questions.

Discussion

Listed below are some useful mental multiplication strategies:

Multiply by 5 by multiplying by 10 then halving

Multiply by 20 by multiplying by 10 then doubling (or vice versa)

Multiply by 50 by multiplying by 100 and halving (or vice versa)

Multiply by 11 or 9 by multiplying by 10 and adding or subtracting

Multiply by 21 or 19 by multiplying by 20 and adding or subtracting

Understanding division problems

The Open University — Wed, 01 May 2019 17:18:55 GMT

Many learners (and adults) have problems with division because they do not exactly understand what it means. A straightforward division can be ambiguous without the context of the problem. Learners need to understand the difference between sharing and grouping which are the two main types of division problem. For example:

can mean there are 12 sweets shared between 3 people, so they each get 4 sweets.
12 has been divided (or shared) into 3 equal parts of 4.
12 ÷ 3 can mean that I have 12p, so how many 3p sweets can I buy? In this case, you can count up in threes to find how many sweets you can buy. You are counting up in groups of 3, (i.e. 3, 6, 9, 12). So there are four 3s in 12.

Division by partitioning can be a useful strategy

The Open University — Wed, 01 May 2019 17:18:55 GMT

Partitioning the number into a multiple of the divisor plus the remainder is a useful way if a learner is familiar with their multiplication tables. For example, 84 ÷ 7:

Activity _unit2.2.4 Activity 6 Division by partitioning

Allow 10 minutes

Calculate 136 ÷ 4 using the partition method. Then watch Video 2 for a demonstration of one way to do this.

Download this video clip.Video player: Video 2 Dividing 136 by 4 using partitioning >

Video _unit2.2.2 Video 2 Dividing 136 by 4 using partitioning

Transcript: Video 2 Dividing 136 by 4 using partitioning

INSTRUCTOR:

I'm going to work out 136 divide by 4 using the partition method. That means I'm going to split 136 into parts. And then I will work out how many 4's go into those parts.

I know that 4 goes into 100, so I'm going to split 136 into 100, and then the amount left over is 36. 4's into 100, there are 25 of these. And then into 36, 4 goes 9 times. So altogether, I've got 25 4's and 9 4's. That makes 34. Finally, 136 divided by 4 is 34.

End transcript: Video 2 Dividing 136 by 4 using partitioning

Show transcript|Hide transcript

Skip transcript: Video 3 Demonstrating formal written methods for addition

Video _unit2.2.2 Video 2 Dividing 136 by 4 using partitioning

Interactive feature not available in single page view (see it in standard view).

Multiplication tables

The Open University — Wed, 01 May 2019 17:18:55 GMT

While there is no doubt that knowing the multiplication tables is a useful skill, learning by rote without an understanding of the concept of multiplication and of division is less than useful. Mental maths, which includes multiplication and division, helps learners to form the appropriate concepts.

Many successful mathematicians do not know their tables by rote. However, they do know how to work them out as needed.

Developing mental arithmetic with your learners

The Open University — Wed, 01 May 2019 17:18:55 GMT

Practise frequently with your learners. The beginning and end of lessons are often good times to practise mental arithmetic – maybe spend two or three minutes doing this.

Starter activities could include working out mental arithmetic questions as part of a quiz, an activity, etc. Make it into a game.

Do ask learners to say how they worked something out. They could tell their method to the person sitting next to them. Or they could tell their method to the class.

Validate mental calculations. Children will not see the point in having to write down a method of arithmetic for a calculation they can do in their head. Nor should they have to: mental calculation is a perfectly good way of working out the answers to number questions, so encourage its use.

3 Developing formal written calculation methods

The Open University — Wed, 01 May 2019 17:18:55 GMT

This course mainly focuses on teaching children in the middle school years, so they have probably met all of the four rules (addition, subtraction, multiplication and division) and been taught formal methods of calculation. This does not guarantee that learners have learned these methods effectively! Nor is there any guarantee that if a learner can operate a method and get lots of right answers that they understand how the method works.

As a teacher, therefore, you are striving to teach effective methods, procedural understanding of how those methods work, and conceptual understanding of the mathematics which underpins the methods.

3.1 Formal written methods of addition

The Open University — Wed, 01 May 2019 17:18:55 GMT

Of the four basic arithmetic operations, the addition of whole numbers and associated written methods of addition tend to be reasonably well grasped. It is the other three operations where learners have difficulties.

Activity _unit2.3.1 Activity 7 Reflecting on two methods for adding three-digit numbers

Allow 10 minutes

Watch Video 3 on formal written methods and then reflect on the difficulties a learner might experience when using either of these methods.

Download this video clip.Video player: Video 3 Demonstrating formal written methods for addition >

Video _unit2.3.1 Video 3 Demonstrating formal written methods for addition

Transcript: Video 3 Demonstrating formal written methods for addition

INSTRUCTOR:

I'm going to work out 426 plus 378. First method I'm going to use will actually partition each number into its hundreds, tens, and units. So 426 is 400 and 20 and 6, and then 378 is 300 and 70, and 8.

And then if I combine the hundreds, that makes 700. If I combine the tens, that gives me 90. And if I combine the units, that's 6 plus 8, is 14. Then I simply add all of those up-- 790, 800, 804. So altogether, that comes to 804.

The other method would be to put 426 and 378 one underneath the other, using the column method. So if I write 426, 378 will go underneath like that so that the units are in the column, the tens are in a column, the hundreds are in a column. Add those up.

6 and 8 makes 14, so I write down the 4 and carry the 1. 2 and 7 makes 9, and then 1 makes 10, so I put down the 0 and carry the 1. 4 and 3 are 7, and 1 makes 8, so that's 804.

The column method is quicker, and may be more efficient. It relies on the digits being placed into columns-- the units in one column, the tens in another, the hundreds in another. And after we've lined up the digits, each of the numbers is added in their colours.

We're effectively treating each number as a string of digits, whereas in the first method, we're actually using the meaning of the 4 in 426 as 400, and the 2 meaning 20, and the 6 meaning six units-- and then similarly for the other number. But it is, obviously, a more chunky method, and not quite so quick to do. But they both come up with the same answer.

End transcript: Video 3 Demonstrating formal written methods for addition

Show transcript|Hide transcript

Skip transcript: Video 4 Choosing the best method for subtraction

Video _unit2.3.1 Video 3 Demonstrating formal written methods for addition

Interactive feature not available in single page view (see it in standard view).

Discussion

If learners have developed mental calculation skills then these will be built on when moving to written addition methods. If learners have not developed good mental calculation skills, using written methods will be much more difficult.

If you think back to the early years, remember that learners operate addition by counting out the sets of objects and then combining the sets. If they have not progressed from this stage to being able to add up abstract numbers mentally then adding large numbers using a written method will be cumbersome.

A learner who struggles to add 6 + 8 and who uses their fingers or makes marks on paper (and I have seen both these activities used by secondary school pupils) will struggle to add 60 + 80. If they only operate addition by counting sets of objects then they are unlikely to have developed a robust concept of place value and therefore no appreciation that 6 tens + 8 tens gives us 14 tens. This is why it is so important that learners build a firm foundation of understanding in the early years. It also implies that expecting learners to move on to the next stage (e.g. of doing formal written addition before they can mentally add abstract numbers) is not appropriate. As a teacher, you may need to ‘go back to basics’ with learners who are in this position.

3.2 Formal written methods for subtraction

The Open University — Wed, 01 May 2019 17:18:55 GMT

Subtraction is experienced as being more difficult than addition. Subtraction is the inverse of addition and knowing this can help learners to use complementary addition in informal methods of subtraction.

Video 4 demonstrates the decomposition method (or column method) of subtraction and the empty number line method which uses complementary addition. This latter method is a visible example instantiation of the method which many people use mentally.

Download this video clip.Video player: Video 4 Choosing the best method for subtraction >

Video _unit2.3.2 Video 4 Choosing the best method for subtraction

Transcript: Video 4 Choosing the best method for subtraction

INSTRUCTOR:

So a lot of learners in school are taught to subtract in columns. So for example, if we've got 86 and I want to subtract 39, those would be put in columns, so the units are underneath each other, the tens underneath each other. And I'm going to subtract. Now, I can't do 6 take away 9. So I'm going to go over to the 8, that's eight 10s. I'm going to take one away, which will leave me with 7.

And then that 10 over here becomes 10 units. So together that makes 16 units, take away 9. 16 take 9 is 7. And then 7 take 3 is 4. So that seems quite straightforward. That's known as the decomposition method because we decompose the 8 into 7 tens and then 10 units over here.

This can sometimes be quite problematic. Sometimes, learners can just simply do the 9 take 6 and provide me with 3. And then 8 take 3 would be 5. So that's the very common mistake. They know they've got to subtract something so they just take the smaller from the larger and don't take any notice of whether the number's actually on the top or not.

Consider this. If I've got 16 take away 9 and I tried to do that using composition composition, I can't do 6 take 9. So I come here, take a 10 away, that leaves me with none. I put the 10 there is 10 units. That's now 16 take 9. And actually, I've got the same problem I had before, haven't I? So 16 take 9 doesn't really make terribly much sense using the decomposition method.

Or again, if I had a number like this 1,004 and supposing I wanted to subtract 99 and I wanted to use the decomposition method, I can't do 4 take 9. So I go to that, but there aren't any n or to there. So I've got to go all the way to the thousands. I'll take that 1,000 off and leaves with 0. Then I can put that there as 10 hundreds. I can now take 1 away from there and leave it with 9.

I've now got 10 tens. And I'll take one of those off and leave me with 9. And then I've got that 10 units. finally there. 14 take nine is 5. 9 take 9 is nothing. 9 take nothing is 9. And there isn't anything left in the thousands. Now I've done that accurately, but it is very messy. And it's fraught with difficulties, because you've got to go all the way to the thousands to decompose and put the numbers across.

So in the both of these cases, it would actually be better to think about those in a different way. Certainly, if we think of the number line, which was demonstrated in the previous section-- so if I was thinking about nine and what I need to add to make 16 I could say to myself, well to get to 10, I've added 1. And then 10 to 16, I've added 6. So that's 7. 16 take 9 is 7.

And over here, I could also use a number line. So if I were to decide what I need to add to 99 to make 1,004, so going from 99 to the next 100, I've added 1. 100 to 1,000, I've added 900. I should set out 1 there. And finally, that last little bit I added 4. So altogether, I've added 900, 4, and 1. So I've added 905. So 1,004 take 99 is 905.

Both of these subtractions made much better sense to do using a number line. Or you might think for this one, 99 is quite close to 100, so take 100 off and then add 1 back on. So 1,004 take off 100, 904, add 1 back on 905. So what it shows is that it rather depends on the problem you're trying to solve as to which is the best method to use.

End transcript: Video 4 Choosing the best method for subtraction

Show transcript|Hide transcript

Skip transcript: Video 5 Two methods for long multiplication

Video _unit2.3.2 Video 4 Choosing the best method for subtraction

Interactive feature not available in single page view (see it in standard view).

Activity _unit2.3.2 Activity 8 Choosing the most appropriate method of subtraction

Allow 8 minutes

Consider the following subtractions and decide whether it is more appropriate to use the decomposition method or the empty number line method.

a.
b.
c.The length of a journey which starts at 9.55 a.m. and ends at 11.05 a.m.

Discussion

If the numbers in the subtraction are close together then it makes more sense to ‘count up’ from the smaller to the larger, the method known as known as complementary addition. This is especially useful if the numbers bridge over a 10s or 100s sort of number or over the hour in a time calculation.

3.3 Multiplication and division

The Open University — Wed, 01 May 2019 17:18:55 GMT

The next video demonstrates two methods for long multiplication and discusses the pros and cons of each method.

Download this video clip.Video player: Video 5 Two methods for long multiplication >

Video _unit2.3.3 Video 5 Two methods for long multiplication

Transcript: Video 5 Two methods for long multiplication

INSTRUCTOR:

I am going to find the result of multiplying 74 by 36 first of all by using the grid method. This links to finding the area of a rectangle 74 units long and 36 units wide. I'm going to partition 74 and 36 into that tens and units, which will mean splitting the length of the rectangle into 70 and 4 and the width of the rectangle into 30 and 6. I'm now then to find the areas of the four parts of the rectangle. So the top left, 30 times 70, three 7's are 21.

So 30 times 70 is 2,100. Top right, 30 times 4 is 120. Bottom left six times 70 is 420. 6 times 4 is 24. So the area of the rectangle is the area of those four parts added up. I'll work out some of the numbers on the top row, that's 2,220. And the bottom row will be 444. And if I add those two up, I'll get the answer to 74 times 36 which is 2,664.

The beauty of the grid method is that the learner works out four products for a two digit by two digit calculation. And if any mistakes are made, it is easy to check where those mistakes are. The learner is also dealing with those numbers as the sum of their parts, 70 and 4, 30 and 6 in this case.

Another method of working out the same multiplication would be to use the standard column method. This is the traditional method, which has been taught for decades. So you would think that every adult would be proficient at it but surveys show that very few people in the general population are proficient at using the standard column method. So we'll look at that method now.

The numbers are written one underneath the other, 74, 36, and I put a multiplication at the side to remind us that that's what we're doing. We're going to have three lines of working. First of all, I'll multiply 74 by 6. Six 4's are 24. So I'll put down the 4 and carry the 2 very small in the next column. six 7's are 42 plus the two that I carried makes 44.

Then I come to multiply by the 3, remembering that it is actually 30. So to multiply by 30, I put down the 0 then carry on as if I were multiplying by 3. Three 4's are 12. So I put down the 2 and carry the 1 very small into the next column. Three 7's are 21 plus the one that I carried makes 22. And if I add those two lines up I got 2,664, the same as I did for the grid method. You might notice that they've got the same lines of working.

Now the standard column method is neat and quick if the learner knows how to do it correctly. However, for many learners, this method is fraught with difficulties. It treats each number as a string of digits, unlike the grid method which treats each number as the sum of its parts. This can mean the learner operates the standard column method according to the set of rules, and they can be easily forgotten or confused.

Some of the procedural problems I've seen are learners not knowing what to do with the carrying figures or forgetting to put down the 0 or not putting it in the units column but putting it at the side and getting lost during the working as they don't know their multiplication tables and need to work them out, whereas in the grid method, they still have to work out the multiplication table facts, but at least the other three boxes can wait for them. And there are no carrying figures to worry about.

End transcript: Video 5 Two methods for long multiplication

Show transcript|Hide transcript

Skip transcript: Video 6 Two ways to solve a division problem

Video _unit2.3.3 Video 5 Two methods for long multiplication

Interactive feature not available in single page view (see it in standard view).

Division

Division is the arithmetic operation where learners experience the most difficulty. Division is the inverse of multiplication which means that it is possible to approach division by complementary multiplying. For example, to answer the question 78 ÷ 6 you can ask ‘How many 6s make 78?’.

One way to answer the question in this way is to use the ‘chunking’ method where you add ‘chunks’ of 6 until you reach 78. It saves having to add 6 at a time, which could be laborious. For example:

(you know that 78 is more than 6 lots of 10, so start there)

(you have 18 left to make the 60 up to 78, that’s 3 lots of 6)

Or you could start with 78 and keep subtracting 6 until you have nothing (or a number smaller than 6) left: For example:

SAQ _unit2.3.1

78 ÷ 6 calculating by subtracting sixes

Answer

Whether working up to 78 or down from 78, the method is basically finding how many 6s there are in 78. This method clearly becomes harder with larger numbers and requires knowledge of multiplication tables and good mental arithmetic. It is, however, an intuitive method of dividing.

The traditional method of division is known among teachers as either the bus stop method (because the working looks like a bus stop),or the ‘gazinta’ method (because you ask how many times 6 ‘goes into’ each number).

Figure _unit2.3.1 Figure 5 One formal method of short division

Figure 5 shows that 6 goes into 7 once with 1 left over. So, put the 1 (for the once) above the 7 and the 1 left over next to the 8. Now read it as 6 goes into 18, which is 3. Put the 3 over the 8. This is an easy method to operate if you can remember the procedure. It is not so easy to understand why it works. Yet many learners prefer it to having to use the ‘chunking’ method.

The bus stop method is efficient to use with larger numbers as well, as long as your multiplication tables are good, and you can do some mental calculation. When dividing by two-digit numbers, I prefer to use and to teach a version of this short division method.

Activity _unit2.3.3 Activity 9 Solving a division problem

Allow 10 minutes

Suppose you have 1960 pupils and teachers in a school to ferry to an event in buses which each carry 43 passengers. How many buses will be needed?

So, you are trying to find out how many times 43 goes into 1960 and you need an extra bus for the remainders.

Spend ten minutes ‘having a go’, using whichever method you prefer. Then watch the video.

Download this video clip.Video player: Video 6 Two ways to solve a division problem >

Video _unit2.3.4 Video 6 Two ways to solve a division problem

Transcript: Video 6 Two ways to solve a division problem

INSTRUCTOR:

How many buses do I need to carry 1,960 passengers if each bus carries 43? First, I will begin by using an intuitive method whereby I will add up multiples of 43 until I can carry all 1,960 people.

So I'm going to start by working out how many 10 buses would carry. 10 times 43 is 430. I obviously need a lot more than 10 buses. I will try 4 lots of 10 buses, which is 40 buses. That will be 4 times 430-- I could double and double again, that will give me 1,720 passengers. This is closer to 1,960, but adding another 10 buses will take me well over the 1,960, so I'm now going to add smaller numbers of buses.

I could add 5 buses. That'll be half of 430. So 5 times 43 will be 215. I'm going to add those together, just the last two lines, to give me 45 lots of 43. So that will be 1,935. So that's not quite enough passengers yet can be taken, so I need to add another bus. Let's try 46 buses by adding 43 onto my last figure. So that will give me 1,978. So I can definitely carry all 1,960 people, and they will also be 18 spaces left. So I need 46 buses.

The traditional way to work out how many buses are needed is to do a division. So I'm doing 1,960 divided by 43. I'm going to use a short division method even though I have a two-figure number as a divisor. Years of teaching experience shows me that learners struggle with the long division method with its complicated procedure.

So 43 into 1,960, I'm going to have to work out how many 43's are in 196. So I'm going to write the 43 times table down the side. So two of them will be 86, three will be 129, four will be 172. So that's below 196, but another 43 would take it over. So 43 is into 196. It must be 4, then of course, it's actually 443-- this is 172, so there's a remainder of 24, which I put next to the 0, and read it as 240.

So 43 is into 240, I just need to go a bit further with my 43 times table, add another one of those on. So that's 215. That's just less than 240. Another 43 would take it over, so 43 is into 240, go 5. And of course, five 43's a 215. So there's a remainder of 25.

So we've got an answer-- 45 remainder 25. What does remainder 25 mean? Do 25 people have to walk? We can't have that, so I will order them another bus. So altogether, I will need 46 buses.

End transcript: Video 6 Two ways to solve a division problem

Show transcript|Hide transcript

Skip transcript: Video 7 Decimal number lines with 1, 2 and 3 decimal places

Video _unit2.3.4 Video 6 Two ways to solve a division problem

Interactive feature not available in single page view (see it in standard view).

4 The concept of place value and decimals

The Open University — Wed, 01 May 2019 17:18:55 GMT

Place value underpins our number system, so understanding it is important in being able to calculate with numbers. Even learners in secondary education can be unsure of the place value system, so it is important to provide activities which support their conceptual understanding of it.

4.1 The decimal point and calculations with decimals

The Open University — Wed, 01 May 2019 17:18:55 GMT

The decimal point is a symbol which is placed between the whole part of the number and the fraction part of the number. It signifies that the digit to its left is a units digit and the digit to its right is a tenths digit. All the rest of the digits fall into place to the left and right. Nothing more, nothing less. The decimal point is not some sort of bridge to jump over or obstacle to be overcome but many learners treat is as if it is.

Here is an example of a common mistake.

Since 5 + 5 makes 10, the learner has written 10 in the tenths column, presumably because they do not think that carrying over the decimal point is allowed. They see the decimal point as splitting the numbers into two parts which need to be dealt with separately.

However, a knowledge of place value means that the 1.5 is seen as 1 unit + 5 tenths, which when doubled is 2 units + 10 tenths (convert to 1 unit) which is 3 units.

One way of helping learners to appreciate this is to practise decimal times tables. Here are the first ten multiples of 0.3:

0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, 3.0

It is tempting to say 0.12 after 0.9 but think about what it means in terms of the value of each digit. Bridging over the whole numbers is always the crunch point for the decimal times tables. They provide a useful exercise for helping learners see that numbers are carried into the next column on the left in just the same way as in whole numbers. They also look the same (as in having the same digits) as their related multiplication tables but including the decimal point.

You can think of all numbers as being decimals if you think of whole numbers as having an invisible decimal point after the units digit. You usually only make the decimal point visible if there are digits after it.

Activity _unit2.4.1 Activity 10 Using decimal times tables

Allow 3 minutes

Write out the first ten members of the 1.5 times table. You will know that you have them correct if the last one is 15.0.

4.2 Becoming familiar with decimals

The Open University — Wed, 01 May 2019 17:18:55 GMT

Adding and subtracting decimals can be done using the same methods as adding and subtracting whole numbers as long as the digits are lined up under the correct column headings, keeping the decimal points underneath each other.

Learners can struggle to handle decimals, so it is helpful to use real-life instances with which the learners are familiar.

Numbers with one decimal place are evident on a classroom ruler set out in centimetres (Figure 6).

Figure _unit2.4.1 Figure 6 A ruler showing centimetres and tenths of centimetres

There are 10 gradations between each whole number (of centimetres), so each interval is 1 tenth or 0.1. Rulers are effective number lines which learners have easy access to.

A useful activity to use with learners who struggle with decimals is to ask them to draw a straight line across a page. Next, they should put a mark near the beginning and label it A. Ask them to put another mark 4.2 centimetres along the line from A and to label it B. Next ask them to put another mark 2.9 centimetres along the line from B and label it C. Now ask them to measure from A to C. Then ask how they can check their answer (i.e. add up 4.2 and 2.9). This activity (and you can use more than 2 measurements) is useful in encouraging the accurate use of a ruler and measuring and working with decimals.

Numbers with two decimal places will be familiar to your learners through the use of money. Having said this, today many people use contactless card payments and may not even think about their transactions in terms of exchanges of money. Yet, I often used to say to my learners ‘think of two decimal place numbers as money’ and it helped them to feel they could cope with the calculations.

Now watch Video 7 for more advice on decimal number lines.

Download this video clip.Video player: Video 7 Decimal number lines with 1, 2 and 3 decimal places >

Video _unit2.4.1 Video 7 Decimal number lines with 1, 2 and 3 decimal places

Transcript: Video 7 Decimal number lines with 1, 2 and 3 decimal places

INSTRUCTOR:

Here is a number line from 0 to 10. Let's put the part of the number line between 3 and 4 under an imaginary magnifying glass. Imagine I place a magnifying glass over the section of the number line between 3.1 and 3.2 showing 10 intervals. These numbers can be filled in. We will now magnify the line between 3.17 and 3.18.

As can be imagined, we could go on and look at four decimal place numbers. I hope you can see that as we use more decimal places, the quantities those digits represent are smaller and smaller. When we consider measuring in centimetres, we cannot measure much more accurately than to one decimal place-- possibly two decimal places. This gives some justification as to why we might wish to round many calculations using measurements to one or two decimal places.

End transcript: Video 7 Decimal number lines with 1, 2 and 3 decimal places

Show transcript|Hide transcript

Skip transcript: Video 8 Multiplying and dividing by powers of 10 using a place value diagram

Video _unit2.4.1 Video 7 Decimal number lines with 1, 2 and 3 decimal places

Interactive feature not available in single page view (see it in standard view).

Since we use a number system in base ten, special things happen when you multiply or divide by powers of 10. A place value table is useful for explaining what happens. Now watch Video 8 for more information on using a place value diagram.

Download this video clip.Video player: Video 8 Multiplying and dividing by powers of 10 using a place value diagram >

Video _unit2.4.2 Video 8 Multiplying and dividing by powers of 10 using a place value diagram

Transcript: Video 8 Multiplying and dividing by powers of 10 using a place value diagram

INSTRUCTOR:

Since we use a number system in base 10, special things happen when you multiply or divide by powers of 10. A place value table is useful for explaining what happens. To work out the effect of the calculation 123.45 times 10, move each digit one place to the left, which makes every digit 10 times bigger to give us 1,234.5.

Note that the decimal point is not moving one place to the right. Thinking like this is the instrumental way of thinking about it, are you using rules without reasons? The digits all move one place to the left to become 10 times as big. That is the conceptual way of thinking about it, or how the maths works.

Multiply 23.45 by 100 by moving each digit two places to the left to get 2,345. we do not need to show the decimal point if there are no digits afterwards. So we write 2,345. 3.04 divided by 100 can be found by moving each digit two places to the right to make them 100th the size they were before. We need to add a 0 in the tenths place as a place holder for neatness a 0 in the units place. 3.04 divided by 100 equals 0.034.

What happens to the rule which tells you that to multiply by 10, you must add a 0? 2.4 times 10 equals 24. As we can see, each digit moves one place to the left. So simply adding a 0 on the end does not work with decimal numbers.

However, we know that this method holds for whole numbers. 24 times 10 equals 240. Let's look at the place value diagram. Two 10's plus 4 units is multiplied by 10. Move each digit one place to the left to give us 200 plus four 10's. We need to add a 0 as a place holder in the units column. That is why it works to 0 on the end of a whole number when multiplying by 10.

End transcript: Video 8 Multiplying and dividing by powers of 10 using a place value diagram

Show transcript|Hide transcript

Skip transcript: Video 1 Mathematical cake: a diagrammatic representation of the equivalence of fractions, decimals and percentages

Video _unit2.4.2 Video 8 Multiplying and dividing by powers of 10 using a place value diagram

Interactive feature not available in single page view (see it in standard view).

5 This week’s quiz

The Open University — Wed, 01 May 2019 17:18:55 GMT

Check what you’ve learned this week by doing the end-of-week quiz.

Week 1 practice quiz

Open the quiz in a new tab or window and come back here when you’ve finished.

6 Summary of Week 1

The Open University — Wed, 01 May 2019 17:18:55 GMT

Now that you have completed Week 1 you have learned how early counting leads to early addition, which then leads to subtraction, multiplication and division. Careful development of these skills helps children to achieve a sense of how numbers work, providing a firm foundation for future mathematics learning. You have explored varied methods, both mental and written methods, for adding, subtracting, multiplying and dividing. Finally you have looked at place value and decimal numbers. You should now be able to:

briefly describe how number sense develops in young children
demonstrate a variety of mental, informal and formal written methods of basic arithmetic
understand how place value underpins our number system in base ten, for both whole numbers and decimals.

Next week you will learn about the conceptual basis of fractions and percentages, the pitfalls learners often meet when studying fractions in particular and effective approaches to the teaching of fractions and percentages.

You can now go to Week 2.

1 Equivalence as a concept applied to fractions, decimals and percentages

The Open University — Wed, 01 May 2019 17:18:55 GMT

In mathematics the term ‘equivalence’ is used to mean ‘has the same value’ or ‘can be substituted for’.

Fractions, decimals and percentages each have two main uses:

Fractions, decimals and percentages are numbers, for example, , 3.2, 78% are all numbers and have a position on a number line
Fractions, decimals and percentages can be used to denote a proportion of a quantity, for example we can find of £6, 3.2 of 5 kilograms or 78% of 250 people.

Confident learners may be able to move between the number meaning and the proportion meaning of fractions, decimals and percentages but many learners find this confusing.

Consider the following well-known equivalences:

If you think of these as numbers then their equivalence means that they all occupy the same position on a number line. Their equivalence in this case means ‘has the same value’.

If you think of these as proportions then their equivalence means that finding of a given quantity is the same as finding 0.5 of that same given quantity or 50% of that same given quantity. Their equivalence in this case means ‘can be substituted for’.

1.1 Diagrammatic representations of equivalence

The Open University — Wed, 01 May 2019 17:18:55 GMT

The first video shows a mathematical cake which is shaped like a rectangle and has been cut into ten slices. Learners can draw their own mathematical cake on paper. It can have a varied length but it is useful to ensure that the width is 10 cm, which means each slice is 1 cm wide. Diagrams can be very useful aids to teaching concepts in mathematics.

Download this video clip.Video player: Video 1 Mathematical cake: a diagrammatic representation of the equivalence of fractions, decimals and percentages >

Video _unit3.1.1 Video 1 Mathematical cake: a diagrammatic representation of the equivalence of fractions, decimals and percentages

Transcript: Video 1 Mathematical cake: a diagrammatic representation of the equivalence of fractions, decimals and percentages

INSTRUCTOR:

Mathematical cakes, a diagrammatic representation of the equivalence of fractions, decimals, and percentages. Here is a mathematical cake, which is rectangular in shape and split into 10 equal slices. We'll consider the percentages first the whole cake is 100%. so one slice must be 100% divided by 10, or 10%. No slices is 0%, one slice is 10%, two slices are 20%, three slices of 30%, and so on and one whole cake is 100%.

Next the decimals, the whole cake is 1.0. So each slice must be 1.0 divided by 10, which is 0.1. No slices is 0.0, one slice is 0.1. two slices are 0.2, three slices are 0.3, and so on. And the whole cake is 1.0.

Finally, the fractions, the whole cake is one whole, so each slice must be 1/10. No slices is 0, one slice is 1/10, two slices are 2/10, three slices are 3/10, and so on. And the whole cake is one whole.

Some of these fractions have equivalent simpler fractions. 5/10 is half of the cake. And that can be seen to be equivalent to 0.5 and 50%. 3/10 is 0.3 and 30%. and 60% is equivalent to 0.6 and 6/10, which can be simplified to 3/5.

You can easily read across to find equivalences from the diagram. The fractions, decimals, and percentages shown on the diagram are some of the important equivalences to know. Supposing you wanted to find out 1/4 from this diagram, how would you find where 1/4 is and work out its equivalences in decimals and percentages? What would you do about 3/4?

End transcript: Video 1 Mathematical cake: a diagrammatic representation of the equivalence of fractions, decimals and percentages

Show transcript|Hide transcript

Skip transcript: Video 2 Multiplying fractions using three different approaches

Video _unit3.1.1 Video 1 Mathematical cake: a diagrammatic representation of the equivalence of fractions, decimals and percentages

Interactive feature not available in single page view (see it in standard view).

Activity _unit3.1.1 Activity 1 Finding equivalences to one-quarter and three-quarters

Allow 5 minutes

Look at the mathematical cake diagram which was shown in the video. Use it to find equivalences to and .

Discussion

The mathematical cake diagram can be used to demonstrate the equivalences of percentages, decimals and fractions in tenths.

One-quarter can be found by finding the mid-way point between 0 and one-half. Three quarters would be found mid-way between one-half and one-whole of the cake.

One-quarter is 25% and 0.25.

Three-quarters is 75% and 0.75.

The fraction wall

The Open University — Wed, 01 May 2019 17:18:55 GMT

Figure 1 shows a fraction wall divided into strips of the same length which can be split into equal parts. The fraction wall can be used to identify equivalent fractions.

Figure _unit3.1.1 Figure 1 Fraction wall

Activity _unit3.1.2 Activity 2 Finding equivalent fractions

Allow 5 minutes

Use the fraction wall to find as many fractions as you can that are equivalent to .

1.2 Challenges to understanding the equivalence between fractions

The Open University — Wed, 01 May 2019 17:18:55 GMT

You have been introduced to two diagrams which can help people learn about the equivalence of fractions, decimals and percentages through visualisation.

Activity _unit3.1.3 Activity 3 Thinking of other useful diagrammatic representations

Allow 5 minutes

Can you think of any other diagrams which you have used, or seen in use, which help learners to understand fractions, decimals and percentages?

The concept of equivalence is important in mathematics. Can you think of other areas of mathematics where equivalence is used?

Discussion

Equivalence is an important concept in mathematics. Algebra is a particular area of mathematics where equivalence is important. Equivalent algebraic expressions are different ways of denoting the same relationship.

2 Approaches to teaching fractions

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will meet again the ideas of equivalence in fractions. You will also be introduced to some different approaches to teaching the calculation of fractions, including the use of diagrams.

Activity _unit3.2.1 Activity 4 Where are fractions used in mathematics?

Allow 5 minutes

The topic of fractions is a substantial one in the school curriculum.

List as many of the different learning activities as you can which relate to the topic of fractions.

Discussion

Did you think of any of the following?

finding fractions of amounts
finding equivalent fractions
ordering a list of fractions from smallest to greatest or vice versa
adding and subtracting fractions
multiplying and dividing fractions
and using fractions within other topics:
solving problems using proportion
algebraic equations which involve fractions
calculating probabilities

2.1 What are fractions?

The Open University — Wed, 01 May 2019 17:18:55 GMT

Fractions are numbers made of two integers (positive and negative whole numbers), one acting as the numerator and one acting as the denominator.

(Fractions typically refer to quantities which are smaller than 1. In this case the numerator is smaller than the denominator. Fractions with a numerator larger than the denominator have a value greater than 1. They can also be written as mixed numbers; the whole number part and the fraction part).

Consider the fraction illustrated in the diagram below which shows of a rectangle shaded. The rectangle has been divided into 4 equal parts and 3 of these parts have been shaded.

Figure _unit3.2.1 Figure 2 A rectangle shaded to show three-quarters

Three- quarters of any quantity can be found using the same process. is found (by dividing by 4 to calculate four equal parts), and then multiplied by 3 to give . This is referred to as a part-whole relationship (Figure 2).

However the number can also be taken to mean that 3 has been divided into 4 equal parts and in this case the value of the fraction is equal to the numerator divided by the denominator. This interpretation of the fraction is used when converting fractions to their equivalent decimals. If you input into a calculator you will get 0.75, which is the decimal equivalent to . This takes the meaning of a quotient (the result of dividing one number by another).

Essentially a fraction such as can have two meanings (the whole-part relation and the quotient) and this can cause difficulty in understanding for some students. One way to address this difficulty is to tell students, for example, that means 3 lots of of something, and it can also mean . This is not the only such case in mathematics, where symbolic notation can take on different meanings and which causes difficulties in students’ understanding.

2.2 Ordering fractions

The Open University — Wed, 01 May 2019 17:18:55 GMT

The understanding of equivalence among fractions can also cause a challenge because numbers which effectively have different labels can mean the same quantity of proportion (depending on how they are being used). As an example and are equivalent fractions. They can both be shown to be equivalent to the simplest form of by ‘cancelling down’ i.e. dividing the numerator and denominator in each fraction by the same common factor.

Since the value of the fraction is determined by the relation between the numerator and the denominator it can be difficult to compare the sizes of the two fractions. The usual method when comparing the size of two fractions is to find equivalences which have the same denominator.

Activity _unit3.2.2 Activity 5 Which fraction is larger?

Allow 10 minutes

Find the larger of these two fractions: or .

Discussion

and which is larger?

Both the denominators 3 and 8 are factors of 24 (the least common multiple or LCM) so you can find equivalent fractions whose denominators are 24

= multiplying numerator and denominator by 8

= multiplying numerator and denominator by 3

So is the larger fraction

2.3 Adding and subtracting fractions

The Open University — Wed, 01 May 2019 17:18:55 GMT

Learners are often told that the denominator in the fraction denotes what kind of a fraction it is (halves, thirds, etc.) and that it is only possible to add or subtract the same types of fraction, i.e. the denominator has to be the same number. This is where the difficulty lies as learners struggle to find out which number should be the common denominator and what needs to be done to change each fraction so that they do have the same denominator. Several rules and procedures exist to help learners to add and subtract fractions but, since the conceptual basis is poorly understood, learners typically do not remember the rules and procedures. This often leads to fractions being taught again and again with little success.

It is worth looking at other methods which build on and develop learners’ conceptual understanding of fractions. The next section gives an approach to teaching the adding of fractions which is based on learners’ prior knowledge of finding fractions of a quantity.

Activity _unit3.2.3 Activity 6 Shading a grid

Allow 10 minutes

Try the following activity.

Draw a 6 by 4 grid. Next shade the grid according to this schedule (with no shading overlapping):

Shade in red

Shade in blue

Shade in green

Shade in yellow

Shade in grey

The whole rectangles should have been shaded once the activity is completed.

Discussion

Did your grid look like this, or at least have the same number of parts per colour?

Figure _unit3.2.2 Figure 3 shading fractions of a rectangle

Activity _unit3.2.4 Activity 7 Use the grid to add fractions

Allow 5 minutes

I hope it is clear that each box in the rectangle is since the grid has been split into 24 equal parts. This should help you (and your students) to calculate the following additions of fractions based on the shaded grid.

Discussion

Did you get these answers?

With some experience of adding fractions using this grid shading methods students should begin to see that they are looking for a common denominator which would have been the number of parts in the grid. Subtraction can be carried out in a similar way by thinking about shading parts on the grid and then rubbing some of them out.

2.4 Multiplying fractions

The Open University — Wed, 01 May 2019 17:18:55 GMT

The procedure for multiplying fractions is straightforward but the conceptual understanding is not so easy to develop.

Watch video 2 on multiplying fractions which demonstrates three different approaches.

Download this video clip.Video player: Video 2 Multiplying fractions using three different approaches >

Video _unit3.2.1 Video 2 Multiplying fractions using three different approaches

Transcript: Video 2 Multiplying fractions using three different approaches

INSTRUCTOR:

Multiplying fractions using three different approaches. Suppose we calculate 3/4 times 2/3. Using the procedure of multiplying the two numerators and multiplying the two denominators, we get 6/12, and that cancels to 1/2. Or I could have cancelled numbers top and bottom before multiplying. I can see I've got a 3 on the top and a 3 on the bottom, so I can cancel those 3's. And I've got a multiple of 2 on the top and a multiple 2 on the bottom. 2's into 2 go once, and 2's into 4 go twice. And that leaves me with 1/2.

These procedures seem straightforward, but they don't really build the conceptual understanding which underpins the multiplication of fractions. So let's look at other approaches, which can help build that understanding. The method I'm going to demonstrate now, it helps to remember that in mathematics the word of is interchangeable with multiply. So 3/4 times 2/3 can be thought of as 3/4 of 2/3.

At present, it would be difficult to calculate 3/4 of 2/3. I need to change 2/3 into an equivalent fraction, which is divisible by 4. If I had a numerator of 4, that would be helpful because I can find 3/4 of 4. If I multiply the numerator and denominator in 2/3 by 2, that will give me 4/6, so that will be 3/4 of 4/6. Now 3/4 of 4 is 3, so 3/4 of 4/6 is 3/6, and that's equivalent to a 1/2.

I could display 3/4 times 2/3 as 3/4 of 2/3 on a grid. And a four by three grid would make sense in this case. First, I'm going to shade 3/4 of the grid, so I will shade three out of the four columns. Then, I'm going to shade 2/3, so I will shade two out of the three rows. The overlap is 3/4 of 2/3, and that's 6 out of 12 parts, and that's equivalent to a 1/2.

End transcript: Video 2 Multiplying fractions using three different approaches

Show transcript|Hide transcript

Skip transcript: Video 3 Dividing with fractions

Video _unit3.2.1 Video 2 Multiplying fractions using three different approaches

Interactive feature not available in single page view (see it in standard view).

2.5 Dividing fractions

The Open University — Wed, 01 May 2019 17:18:55 GMT

The standard procedure for dividing by a fraction is one of those where learners can get incredibly confused. ‘Turn it upside-down and multiply’ is the common mantra but which fraction (if there are two) and why does it work?

Division is the most difficult of the arithmetic functions, even for whole numbers. It can help to go back to remembering what division means. The grouping method of division is most useful when dealing with fraction division. Watch Video 3 for some useful approaches to teaching division with fractions.

Download this video clip.Video player: Video 3 Dividing with fractions >

Video _unit3.2.2 Video 3 Dividing with fractions

Transcript: Video 3 Dividing with fractions

INSTRUCTOR:

I'm going to look at the problem 2 divided by 2/3. What does this mean? One understanding of division is that the problem is asking, how many 2/3's are there in 2?

I can work this out by counting up in 2/3's and seeing where I get 2. So if I've got one 2/3 and then add on another 2/3 to that, I'd have 4/3. And if I add on another one, I'll get 6/3.

Now, 6/3 is 2. So the answer to the question 2 divided by 2/3 is 3.

This question was easily solved by counting up in 2/3's, but some questions are not so easy. Consider this question, 1 and 3/4 divided by 1/2, which we could say as, how many 1/2's are there in 1 and 3/4?

If I count up in 1/2's, I'd get 1/2, 1, 1 and 1/2, 2, and so forth. So I can see that 1 and 3/4 is in between 1 and 1/2 and 2. So I expect my answer to be a bit bigger than 3.

I'm going to look at this by drawing a diagram. I'm going to draw 1 and 3/4 circles. So that's one circle and then 3/4 circles.

OK. And I'm going to split those up into halves. So the whole circle will have 1/2 in it and another 1/2. And then this 3/4 circle will be 1/2 here, and then we've got a quarter of a circle here.

So I can see from this that I've got three halves, and I've also got the quarter. Now, that quarter is a quarter of a circle. But we're actually looking at 1/2's. So when we're looking at that, we're actually looking at a half of a half. This quarter is a half of a half.

So we've actually got one, two, three halves, 3/2, and then a half of a half. So 1 and 3/4 divided by 1/2 is 3 and 1/2.

I can also choose to do these calculations using the well-known method of turn it upside down and multiply. So it's a nice, quick, and easy way to do it. But you do have to remember the procedure and how it works.

And it's OK if only the procedure is done properly. And what a lot of learners do is think that it's both the numbers that have to be turned upside down, which obviously is the problem here.

So it's actually the second number which is turned upside down and multiplied. If we look at this one here, so 2 divided by 2/3 is the same as 2 multiplied by 3/2.

OK. So 2 lots of 3/2 is 6/2, which is 3. And in this case, 1 and 3/4 divided by 1/2 is the same as 1 and 3/4 times 2/1, 2/1 being 2. That basically means you're doubling it, so that's 3 and 1/2.

What you're doing when you turn it upside down and multiply is you are multiplying by the reciprocal of the second number. In other words, the number which is 1 divided by it. So 2/3 and 3/2, they actually are reciprocals. Because if you multiply them together, you would get 1.

And similarly here, 1/2 and 2/1 are reciprocals, because if you multiply them together, you get 1. So this is what's happening here. You're multiplying by the reciprocal, which is, as it were, equivalent to dividing by the original number.

End transcript: Video 3 Dividing with fractions

Show transcript|Hide transcript

Skip transcript: Video 4 Using the mathematical cake diagram to find percentages of an amount

Video _unit3.2.2 Video 3 Dividing with fractions

Interactive feature not available in single page view (see it in standard view).

The ‘turn it upside-down and multiply’ method works because turning a fraction upside-down produces its multiplicative inverse. Fractions are unlike the integers in that they have a multiplicative inverse which can be used in fraction division because multiplying is also the inverse of dividing.

In the next section you will meet the mathematical cakes model again as a way to help learners to find percentages of amounts.

3 Approaches to teaching percentages

The Open University — Wed, 01 May 2019 17:18:55 GMT

Percentages are typically easier to handle than fractions as they are effectively fractions with a denominator of 100 (hence ‘per cent’ which means ‘out of one hundred’). This makes it very easy to compare the size of percentages. But remember that, for example, 1% of a large amount could well be bigger than 10% of a small amount.

3.1 Calculating percentages of amounts

The Open University — Wed, 01 May 2019 17:18:55 GMT

When calculating a percentage of an amount, many learners reach for the method of putting the percentage over one hundred and multiplying by the amount over one. As an example, 35% of 60 is calculated below using this method.

This method works if you are good at arithmetic and can remember the procedure! However, it is more intuitive to use some common knowledge: that 10% is equivalent to one-tenth, 1% is equivalent to one-hundredth, 50% is equivalent to one-half, and 25% is equivalent to one-quarter. Other percentages can be worked out from these.

The mathematical cake, which you met in Section 1 on equivalence between fractions, decimals and percentages, also proves useful in helping younger learners to understand the concept of percentages of amounts. It can be used to find multiples of 10% of amounts by imagining the cake cut into ten slices as before. Watch Video 4 to find out about a useful approach to working out percentages of an amount.

SAQ _unit3.3.1

Download this video clip.Video player: Video 4 Using the mathematical cake diagram to find percentages of an amount >

Video _unit3.3.1 Video 4 Using the mathematical cake diagram to find percentages of an amount

Transcript: Video 4 Using the mathematical cake diagram to find percentages of an amount

INSTRUCTOR:

Here is a mathematical cake. Mathematical cakes are split into 10 equal slices. The whole cake is 100%. So each slice is 100% divided by 10, which is 10%. Here is a percentage question. Suppose the whole cake costs 3 pounds. How much would 70% of the cake cost? Well, 70% is seven slices of cake. If we work out how much one slice, or 10%, of the cake costs, then we can find the cost of 70% of the cake.

100% of the cake cost 3 pounds. So one slice costs 3 pounds divided by 10. That's 300 pennies divided by 10, or 30 pence. We can write this as naught pounds 30. One slice cost naught pounds 30, two slices cost naught pounds 60, three slices cost naught pounds 90, four slices costs 1 pound 20, and so on, all the way up the cake. Finally, 10 slices cost 3 pounds.

Now we can read off the answer to 70% of 3 pounds as 2 pounds 10. Work out these other percentages using the mathematical cake-- 40% of 3 pounds. Read across, and that's 1 pound 20. 80% of 3 pounds-- read across, and that's 2 pounds 40. 50% of 3 pounds-- read across, and that's 1 pound 50.

Suppose the cake weighs 500 grammes. We can use the mathematical cake method to work out multiples of 10% of 500 grammes. 10% of 500 grammes is the same as 1/10 of 500 grammes, or 50 grammes. We can label the right-hand side of the cake going up in 50 grammes. No slices weigh naught grammes.

One slice weighs 50 grammes, two slices weigh 100 grammes, and so on all the way up the cake. Finally, 10 slices weigh 500 grammes. Then we can read off the answer to the following questions-- 20% of 500 grammes, 90% of 500 grammes, 30% of 500 grammes. Do these questions for yourself and then click on reveal answers to check your answers.

End transcript: Video 4 Using the mathematical cake diagram to find percentages of an amount

Show transcript|Hide transcript

Video _unit3.3.1 Video 4 Using the mathematical cake diagram to find percentages of an amount

Interactive feature not available in single page view (see it in standard view).

Answer

Once learners have been introduced to the mathematical cake diagram, they will certainly be familiar with finding 10% of an amount and multiples of 10% of an amount. It is also intuitive to find 25% and 50%.

For example, 35% of 60 can be found by doing the following:

(multiplying 10% of 60 by 3)
(finding half of 10% of 60)
(adding the results of finding 30% and 5%)

Activity _unit3.3.1 Activity 9 Find percentages of 60

Allow 10 minutes

A useful exercise to do with your learners is to ask them what percentages of 60 they can find, encouraging them to modify the methods they have previously used for finding simple percentages of amounts. Try this one yourself (Figure 4).

Figure _unit3.3.1 Figure 4 Finding percentages of 60

Discussion

There are many possible questions and answers that you could have posed in the diagram.

Examples could be 50% of 60, 30% of 60, or 25% of 60.

Extend learners’ understanding by posing questions such as: 15% of 60, how could you find that? What about 76% of 60? Or 120% of 60?

Ask learners to pose ‘What percentage of 60?’ questions themselves.

So far, if your learners have been using the methods shown above, they will have found percentages of amounts by making reference to the equivalent fractions.

Decimal equivalents can also be used when calculating percentages of amounts and this is a more helpful form when using a calculator. Referring back to the mathematical cake diagram in Section 1.1, remember that etc.

To convert from percentages to decimals, the percentage has been divided by 100. This follows from an earlier observation about fractions, which stated an alternative meaning of the numerator and denominator, i.e. numerator divided by denominator. In other words:

For example, if calculating 76% of 60, the learner keys into the calculator:

However, it becomes easy to convert 76% straight away to 0.76 and then multiply by 60.

It is worth raising a caution here about using the percentage button on a calculator. Since the percentage button functions in different ways on different makes of calculator , it is best not to use it. Instead, encourage learners to convert the percentage to a decimal and then multiply by the amount.

3.2 Percentage change: using multiplying factors

The Open University — Wed, 01 May 2019 17:18:55 GMT

Consider the following problem.

A customer goes into a restaurant and pays for a meal. Because they had the ‘early bird’ menu, their bill is discounted by 25%. There is also a 10% service charge added to their bill. What difference does it make to the customer's bill if the waiter deducts the 25% and then adds the 10% service charge, or alternatively, adds the service charge and then makes the deduction?

Activity _unit3.3.2 Activity 10 Does the order matter?

Allow 15 minutes

Work out the solution to the above problem in both ways and see what the difference is. Don’t forget that the second percentage change is enacted on the result of the first percentage change, i.e. having deducted 25% from the bill, the 10% service charge is worked out on the resulting discounted bill.

You could choose to calculate the solution of the problem using different amounts for the bill. Or you could prove it once and for all using a general case, which requires you to use algebra, by using a letter to represent the amount of the bill.

Discussion

One form of the solution uses algebra to find the general case:

Let the cost of the meal be c.

For 25% deduction followed by 10% increase:

For 10% increase followed by 25% deduction:

Were you surprised at the result? It does not matter which way round the final bill is calculated because a 25% deduction followed by adding on 10% works out the same as adding 10% first followed by deducting 25%.

To see why this is, consider the use of multiplying factors for each of the percentage changes.

A 10% increase means that, having started with 100% of the amount and adding an extra 10%, you end up with 110% of the amount.

A 25% decrease means that, having started with 100% of the amount and subtracting 25%, you end up with 75% of the amount.

Using decimals for the multiplying factors (although you can also use fractions as the multiplying factors), an elegant solution to the problem can be found as follows:

Subtracting 25% then adding 10% gives c

Adding 10% then subtracting 25% gives c

Both multiplications come to 0.825c because it does not matter which way round you multiply (this is known as commutativity).

In short, a percentage change can be calculated by using the appropriate multiplying factor, which is calculated by considering what has happened to the original 100%.

The use of multiplying factors for percentage change does make the calculations much simpler. However, it may be a difficult concept for younger learners to grasp. If this is the case, it is better to allow learners to work the long way round.

In mathematics it is always best to work with the conceptual understanding that learners have demonstrated and to build on this carefully. Learners can do, understand and succeed when they have grasped the conceptual underpinning for the mathematics they are doing. This was summarised by Richard Skemp (1976)as relational understanding. Otherwise, if there is no understanding, learners will resort to trying to learn rules which are easily forgotten. (Skemp referred to the learning of rules and procedures without a conceptual underpinning as instrumental understanding.)

3.3 Finding the original amount when given the result of a percentage change

The Open University — Wed, 01 May 2019 17:18:55 GMT

It is necessary to understand what percentage change is before this type of problem can be understood.

Finding the original amount from the result of percentage change requires some understanding of what happens when algebra is manipulated. However, first consider two easier examples where intuitive thinking can be sufficient.

Activity _unit3.3.3 Activity 11 25% more sunscreen

Allow 10 minutes

A ‘special offer’ bottle of sunscreen promises 25% more than usual and holds 500 ml. How much does the bottle of sunscreen normally hold?

Discussion

Here is one way to arrive at the solution:

25% more means one-quarter more than the usual amount in the bottle of sunscreen. This means that the special offer holds:

You need to find out how much 100% or 1 whole of 500 ml.

This gives 100 ml. Then multiply by 4 to get the whole of the amount in the normal size bottle, i.e. 400 ml.

You can see that the solution was found by dividing by 5 and multiplying by 4. Remembering that a fraction can also denote numerator divided by denominator, this is equivalent to multiplying by the fraction

This means that one is the inverted (upside-down) version of the other and, if they are multiplied together, you get 1.

It also works by using the equivalent decimals 0.8 and 1.25, which can be checked on a calculator.

Activity _unit3.3.4 Activity 12 A coat is reduced in a sale

Allow 10 minutes

A coat is reduced in a sale by 10% and the sale price is £81. What was the original price of the coat?

Have a go at this problem yourself.

Discussion

The sale price is 90% of the original price and you need to find 100% of the original price.

90% of the original price = £81

10% of the original price = £9 (dividing by 9)

100% of the original price is £90 (multiplying by 10)

3.4 Using multiplying factors to find an original amount given the result of a percentage change

The Open University — Wed, 01 May 2019 17:18:55 GMT

Now consider the same problem using algebra.

The sale price of a coat is £81 and this is the result of a 10% discount. Let p be the original price of the coat.

10% discount leaves 90% or 0.9 of the price

You know that 0.9 p = 81

Divide both sides by 0.9 to get p

So, the original cost of the coat is £90.

This method can be used with much harder numbers and it is a simpler and more elegant way of reaching the solution. However, learners need to understand the method and why it works. This is known as procedural understanding. If learners do not have this procedural understanding then the method becomes a rule to be learned and there are too many such rules in mathematics if your approach is to learn rules rather than develop conceptual understanding.

4 This week’s quiz

The Open University — Wed, 01 May 2019 17:18:55 GMT

Check what you’ve learned this week by doing the end-of-week quiz.

Week 2 practice quiz

Open the quiz in a new tab or window and come back here when you’ve finished

5 Summary of Week 2

The Open University — Wed, 01 May 2019 17:18:55 GMT

After completing Week 2 you have met some mathematical diagrams which can be used to represent fractions, percentages and decimals. You will have seen how such diagrams can be effective in helping to:

demonstrate equivalence in fractions, decimals and percentages
show how to calculate fractions and percentages of quantities
support methods for addition, subtraction, multiplication and division of fractions.

An important message from Week 2 is that it is important to build on learners’ understanding of how mathematics works; conceptual understanding, and of how mathematical methods work; procedural understanding.

Next week you will build on the learning from week two when you learn about proportional reasoning, one of the important ideas from mathematics.

You can now go to Week 3.

1 Developing proportional reasoning and an appreciation of multiplicative structure

The Open University — Wed, 01 May 2019 17:18:55 GMT

Proportional reasoning involves thinking about relationships and making comparisons between two or more quantities using multiplicative reasoning. It is a complex way of thinking which requires learners to think about numbers and values in relative, rather than absolute, terms.

Proportional reasoning is a hugely important part of school mathematics as it underpins many other topics. For instance, it is instrumental in working with fractions, decimals and percentages, as well as similarity and enlargements. Proportional reasoning also assists learners when working on other topics throughout the mathematics curriculum, from simple probability to problems involving trigonometry.

In addition to being relevant to several topics in the mathematics curriculum, proportional reasoning also has many practical applications. It is used to calculate best buys, to work with scale drawings and maps, to adjust recipes or create particular concentrations of mixtures and solutions and to perform currency conversions.

1.1 Additive versus multiplicative thinking

The Open University — Wed, 01 May 2019 17:18:55 GMT

For many young learners, comparisons are described in additive terms and groups are compared used subtractive language.

This form of reasoning is based on additive thinking.

For example, in both images in Figures 2 and 3 there are some green and pink cubes. The ratio of green cubes to pink cubes, expressed as G : R, remains the same in each picture.

Figure _unit4.1.1 Figure 2 Green and pink cubes

Figure _unit4.1.2 Figure 3 More green and pink cubes

If a learner is using additive thinking, they will describe the change from 5 to 15 green cubes as an addition of 10, and the change from 1 to 3 pink cubes as an addition of 2.

When a learner uses multiplicative thinking, they will describe the same changes as multiplying by 3.

Being able to describe proportional situations using multiplicative language is an indicator of proportional reasoning.

Activity _unit4.1.1 Activity 1 Growing dogs

Allow 5 minutes

Look at Figure 4 and then consider the following.

Ronnie, the miniature dachshund, weighed 2 kg as a puppy.

Now Ronnie weighs 5 kg.

Ralph, the French bulldog, weighed 9 kg as a puppy.

Now Ralph weighs 13 kg.

Which dog has grown more?

Figure _unit4.1.3 Figure 4 Ronnie and Ralph as puppies and adults

Activity _unit4.1.2 Activity 2 Reflecting

Allow 5 minutes

Consider the problem above. How do you think learners may respond to this question?

Discussion

If learners think in absolute terms, or using additive thinking, they may suggest that Ralph has grown more because his weight has increased by 4 kg whereas Ronnie’s weight has increased by only 3 kg.

If learners think in relative terms, or using multiplicative thinking, they may suggest that Ronnie has grown more because his weight has more than doubled and Ralph’s weight has less than doubled.

Activity _unit4.1.3 Activity 3 Speed on a cycling track

Allow 10 minutes

Two cyclists, Ava and Isaac, are cycling equally fast around the cycling track. (Figure 5)

Ava began cycling before Isaac arrived at the track and had completed 9 laps when Isaac had completed 3.

When Ava has completed 15 laps, how many laps will Isaac have completed?

Figure _unit4.1.4 Figure 5 Ava and Isaac cycling

Activity _unit4.1.4 Activity 4 Reflecting

Allow 5 minutes

How did you approach the task in Activity 3?

Discussion

It is easy to fall into the trap here of taking a proportional approach, resulting in Isaac completing 5 laps, when actually this task requires additive reasoning.

The key words here are ‘cycling equally fast’. Ava and Isaac are travelling at the same speed, so it is not a proportional relationship. Ava had a head start, so completed an extra 6 laps, meaning that when Ava has completed 15 laps, Isaac will have completed 9 laps.

1.2 When to use multiplicative thinking

The Open University — Wed, 01 May 2019 17:18:55 GMT

It is important for learners to distinguish between situations which require additive reasoning and those which require multiplicative, or proportional, reasoning.

The following example offers some possible discussion points for learners.

Activity _unit4.1.5 Activity 5 Questioning learners

Allow 10 minutes

As you read the problem below, think of some questions you could ask learners to promote analysis of each situation in additive and multiplicative terms. Make a note of the questions you could ask in the box below the task.

Using boxes of eggs to create questions for the classroom

Each of the cartons in Figure 6 contains some white eggs and some brown eggs. Which has more brown eggs?

Figure _unit4.1.5 Figure 6 Brown and white eggs

Interactive content appears here. Please visit the website to use it

Discussion

Possible questions to ask learners

How many brown eggs are there in each box?
How many brown eggs are there in comparison to white eggs in each box?
What is the relationship between brown and white eggs in each box?
What is the fraction of brown eggs in each box?
What is the ratio of white to brown eggs in each box?

Asking learners questions such as ‘how many brown eggs are in each box?’ encourages learners to reason additively.

Discussions about relationships lead learners to think proportionally.

Depending on the age and experience of the learners, discussions can move on to comparing fractions, ratios or percentages as ways to represent the proportion of brown eggs in each box.

1.3 Developing learners’ proportional reasoning

The Open University — Wed, 01 May 2019 17:18:55 GMT

Teaching basic algorithms for solving problems involving ratio and proportion will not develop learners’ proportional reasoning on their own.

Learners need their conceptual understanding of proportion to be flexible so that they can apply it to the many varied situations in which proportional reasoning is required.

Working on repetitive exercises can support learners in remembering methods for solving particular types of ratio and proportion questions. But it does not develop their proportional reasoning or prepare them for unfamiliar problems. Instead, learners need to be exposed to a variety of different problems and approaches.

You will consider a range of problems for learners involving ratio and proportion in Section 2 of this week.

1.4 Using concrete resources

The Open University — Wed, 01 May 2019 17:18:55 GMT

It is important to give learners of all ages a variety of concrete experiences connected with proportional reasoning to encourage them to compare, make conjectures, find relationships and generalise their learning.

Young learners may start with weighing and measuring objects in the classroom, and discussing comparisons between amounts.

Activity _unit4.1.6 Activity 6 Sharing money

When asked to share out £24 between two cousins in the ratio of 3:1, learners can use counters to represent the pounds (Figure 7). How can you use counters to demonstrate the solution to this problem? Write responses in the box below.

Figure _unit4.1.6 Figure 7 Using counters to share out in a ratio

Interactive content appears here. Please visit the website to use it

Discussion

Grouping counters in groups of 1 and 3 is one way to work on this problem.

By grouping counters in this way, learners share out all the counters in the ratio of 1:3.

They can then look at how many of each colour there are altogether. Noticing that there are 6 groups of 3 and 6 groups of 1 would result in finding a total of 18 blue counters and 6 yellow counters, representing the £18 and £6 the cousins will each get.

Another way to approach this problem using counters is to create boxes (on paper or on mini-whiteboards) to represent the ratio. In this case, since the ratio is 3:1, you will need 3 boxes for the first cousin and 1 box for the second cousin.

The counters, which represent pounds, are then shared out equally between the 4 boxes. The first cousin is given 3 boxes, so receiving £18, and the second cousin is given 1 box, so receiving £6 (Figure 8).

Figure _unit4.1.7 Figure 8 Using ratio boxes with counters

This approach can also be used with pens on mini-whiteboards. Learners need to create the boxes to represent the ratio and then put a mark in every box until they get to the total, in this case 24 (Figure 9).

Figure _unit4.1.8 Figure 9 Using ratio boxes on mini-whiteboards

This imagery can then support learners in moving towards a more abstract concept of sharing amounts into a ratio. In other words, it can help them to understand that in a ratio of 3:1 there are 4 equal parts in total which the £24 needs to be divided equally between.

Note: When moving on to ratio problems involving fractions and decimals, using manipulative resources can become problematic. It is sensible to move onto more abstract methods with your learners before introducing these kinds of problems.

Teaching idea: Cooking with ratios

The Open University — Wed, 01 May 2019 17:18:55 GMT

Why not incorporate some creative cooking into a maths lesson or some maths into a cooking lesson?

Figure _unit4.1.9 Figure 10 Rice crispy cakes

To make 12 rice crispy cakes you need:

60 g unsalted butter
3 tablespoons golden syrup
1 x 100 g bar of milk or dark chocolate
90 g Rice Krispies^©.

Before making the delicious treats, learners could first work out how much of each ingredient they will need to make, say, 6, 24, 36 or more challenging amounts such as 30 or 50.

Decorating biscuits with icing is an even simpler recipe which could be used with younger learners.

1.5 Moving to abstract thinking and using pictorial representations

The Open University — Wed, 01 May 2019 17:18:55 GMT

Proportional reasoning does not always involve physical objects, so learners need to be able to abstract their thinking about ratio and proportion.

When moving from concrete resources to more abstract thinking, it can be helpful to encourage learners to draw diagrams to support their thinking.

Activity _unit4.1.7 Activity 7 An example of a proportionality problem

Allow 5 minutes

If it takes Sakina 30 minutes to read 45 words, how many words could she read in 60 minutes?

Discussion

Minutes can be compared to minutes. Here 60 minutes is twice 30 minutes.

This multiplicative relationship can then be applied to the relationship between pages. This is known as a scalar comparison.

A pictorial representation of this approach is shown below.

Figure _unit4.1.10 Figure 11 A scalar comparison between minutes and pages

Alternatively, pages can be compared to minutes. This is a proportional comparison.

Here, 45 is 1 times 30, and this relationship can then be related to the 60 minutes.

A pictorial representation of this approach is shown below.

Figure _unit4.1.11 Figure 12 A proportional comparison between minutes and pages

Note that both approaches give the same result.

In the next section you will look at problem solving with proportional reasoning and discuss misconceptions that often arise.

2 Problem solving using proportional reasoning, including common misconceptions

The Open University — Wed, 01 May 2019 17:18:55 GMT

Learners in the middle years often struggle with proportion problems. Research consistently highlights learners’ difficulties with proportion and proportion-related tasks and application.

There are many common misconceptions that arise within the topic of ratio and proportion and it is important to be aware of these in order to plan to address these in your teaching.

2.1 Problem solving using proportional reasoning

The Open University — Wed, 01 May 2019 17:18:55 GMT

Proportional reasoning questions cover a huge variety of real-life applications, from best buys to map scales.

Maps and scales

Maps and scale drawings use proportionality. In order for a map to be useful, it needs a scale which represents how much smaller it is than the place it represents. This means that journey distances can be accurately calculated.

Activity _unit4.2.1 Activity 8 Example problem: model boat

Allow 5 minutes

A scale model of a sailboat is made to a scale of 1:20 (Figure 13).

a.If the model boat is 17 cm wide, how wide is the actual boat?
b.If the boat has a sail of height 5 m, how high is the sail on the model?

Figure _unit4.2.1 Figure 13 A scale model boat

Discussion

a.The actual boat is 20 times larger, so it will be 340 cm or 3.4 m wide.
b.The model boat is 20 times smaller, so the sail will be 0.25 m or 25 cm high

Teaching idea: School or home map

The Open University — Wed, 01 May 2019 17:18:55 GMT

Learners can create scale maps of their school or home by taking measurements using trundle wheels, measuring tapes and metre rulers. Then, using an appropriate scale, they can produce accurate scale drawings or maps.

Alternatively, learners can calculate how far away different countries are by measuring distances on scale maps in a school atlas (Figure 14).

Figure _unit4.2.2 Figure 14 Maps and an atlas

Best buys

The Open University — Wed, 01 May 2019 17:18:55 GMT

Having proportional reasoning helps to ensure you get the best deals when shopping, particularly when it comes to offers and multi-buys. Often deals are misleading and larger ‘value’ packs can work out to be more expensive. It is important to be able to compare prices properly in order to make smart choices.

Activity _unit4.2.2 Activity 9 Example problem: washing-up liquid

Allow 5 minutes

Two shops, Fast-Save and Super-Mart, sell the same brand of washing-up liquid in the same size bottles (Figure 15).

At which shop is the washing-up liquid the best value for money?

Figure _unit4.2.3 Figure 15 Best buys

Discussion

You need to be able to compare the two prices, so you need to find the price at each shop for a common number of bottles.

One approach to this problem is to find a common multiple of 5 and 6. In this case, 30 is a sensible choice as it is the lowest common multiple of 5 and 6.

The price for 30 bottles at Fast-Save is £17.10 (6 £2.85).

The price for 30 bottles at Super-Mart is £19.50 (5 £3.90).

An alternative approach would be to find the unit price for 1 bottle of washing-up liquid at each shop. This would involve dividing the cost by 5 and 6, respectively.

Exchange rates

The Open University — Wed, 01 May 2019 17:18:55 GMT

A common use for proportionality is currency exchange rates. When you change your money at the Post Office or travel agent ‘commission free’, they will use a different exchange rate when they sell you foreign currency to when they buy it, thus making a profit.

Activity _unit4.2.3 Activity 10 Example problem: Japanese yen

Allow 5 minutes

Zoë is going on holiday from England to Japan.

Before leaving, she changes 1500 pounds sterling (GBP) into Japanese yen (JPY).

The exchange rate is 1 GBP = 177.415 JPY.

Calculate the amount of foreign currency Zoë takes on holiday.

Discussion

This question is quite straightforward since you are given the unit rate for £1 in yen. All that is required here is to multiply the number of pounds by the rate, giving 266 122.50 yen.

If the rate was given in another form, for instance: 20 GBP = 3548.30 JPY, another step would be needed to calculate the unit rate.

Mixtures and solutions

The Open University — Wed, 01 May 2019 17:18:55 GMT

Proportionality is used when producing mixtures and solutions. For example, to make concrete you need 1 part cement, 3 parts sand, and 3 parts aggregate.

This is a ratio of 1:3:3, or it can be said that the proportion of cement is 1/7.

Activity _unit4.2.4 Activity 11 Example problem: orange drink

Allow 10 minutes

A bottle contains 750 ml of concentrated orange squash.
It is enough to make fifteen 250 ml glasses of diluted orange drink.
How much water is needed to make 10 litres of this drink?

Discussion

This question tackles proportion in a real-life context. It is a multistep problem as, first, the total amount of diluted drink needs to be calculated before then comparing ratios.

This problem comes from the Nrich website. Suggested approaches and solutions can be found via the link in the ‘Further reading’ section.

Teaching idea: A giant’s hand investigation

The Open University — Wed, 01 May 2019 17:18:55 GMT

One way to engage learners in working on ratio and proportion is to leave a ‘giant’s handprint’ on the classroom wall for them to find (Figure 16).

The learners are then asked to investigate the question ‘How big must the giant be?’

Box _unit4.2.1

Note: Week 8 of this course looks at how you might conduct statistical investigations with learners. You might find it useful to read that section before carrying out this investigation in your classroom.

Figure _unit4.2.4 Figure 16 A giant’s handprint

Activity _unit4.2.5 Activity 12 Reflecting

Allow 10 minutes

a.What do the learners need to know?
b.What do they need to find out?
c.How does this problem relate to proportion?

Discussion

The learners will need to measure the ‘giant’s hand’ and use proportion to calculate the size of the giant.

The idea behind this activity is that learners will investigate the proportions found in the human body. One way to do this is to collect data from the whole class, with each learner measuring their hand size (length, width or both) and height. This is a hands-on, practical activity which will help learners to get a sense of what proportionality means.

Whole-class discussions looking at the class data set may draw out some conclusions about the proportional relationship between hand size and height (or other measures: foot size, arm span, etc.). These relationships can then be related to the giant’s hand measurements to calculate the height (and foot size, arm span, etc.) of the giant. Different learners may come to slightly different conclusions, so encourage them to explain their answers. This will help to develop their proportional reasoning.

Box _unit4.2.2

Note: in exploring the proportions in the human body, you might like to discuss the golden ratio with your learners. This idea was briefly discussed in the Introduction to this week.

Study note _unit4.2.1

Some teaching ideas based around investigating the golden ratio can be found on the Nrich website.

2.2 Common misconceptions with proportional reasoning problems

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section, you are offered several problem-solving tasks relating to proportional reasoning. As you read through each problem, consider how you would approach each task yourself and think about the issues, and possible misconceptions, that might arise for learners as they work on each problem.

Proportional problem 1: Soup

The Open University — Wed, 01 May 2019 17:18:55 GMT

Here is a recipe for vegetable soup (Figure 17).

Vegetable soup (serves 2)

200 g chopped mixed vegetables (e.g. onion and carrot)
300 g potato
700 ml vegetable stock
1 tablespoon olive oil.

Zoë makes the soup using 600 g of mixed vegetables.

What weight of potato should she use?

Figure _unit4.2.5 Figure 17 Vegetable soup

Activity _unit4.2.6 Activity 13 Reflecting

Allow 10 minutes

Look at this learner’s incorrect response below.

Zoe needs 200 g of chopped vegetables in the original recipe.
Now she needs 600 g of chopped vegetables.
200 g + 400 g = 600 g
She needed to add 400 g of chopped veg, so she must need to add 400 g of potato.
So, it must be 300 g + 400 g = 700 g of potatoes.

How can you explain the thinking behind this approach?

Discussion

This is a common approach to this type of problem, known as the constant difference strategy. The learner seems to be thinking in absolute terms. They have identified that the difference between the amount of veg and potato in the original recipe is 400 g, so have kept this difference constant by adding 400 g to the new amount of veg, rather than thinking about the relationship between the ingredients. This is an example of additive reasoning in place of multiplicative reasoning, which was discussed in Section 1 of week 3.

It is challenging for learners to identify when they should be thinking proportionally rather than additionally. Using diagrammatic representations of ratios and having discussions related to the context of the problem can support learners in moving from additive to multiplicative reasoning.

Proportional problem 2: Sharing the workload

The Open University — Wed, 01 May 2019 17:18:55 GMT

It takes 3 workers 8 days to complete the foundations of a new building (Figure 22).

How long would it take if there was an extra worker?

Assume that all the workers will work at the same rate.

Figure _unit4.2.6 Figure 18 Building a brick wall

Activity _unit4.2.7 Activity 14 Reflecting

Allow 10 minutes

Look at the learner’s incorrect response below.

How could you explain to a learner that this approach is incorrect?

There are 8 days of work completed by 3 workers.
So, 8 ÷ 3 = 2 2/3 days of work per person.
Now there are 4 workers, so:
4 2 2/3 = 10 2/3 days.

Discussion

This requires some logical thinking, relating the answer back to the original problem.

If it takes 3 workers 8 days to complete the job, would it take 4 workers more time to complete the same job?

No, it would take them less time, so this answer cannot be correct.

If learners have been working on ‘sharing in ratio’ problems, they may go into automatic mode and try to use the same approach with these types of problems, causing them to make mistakes. When working on problems with ratio and proportion, the context of the problem is important.

It will take 4 workers 6 days to complete the same work.

Proportional problem 3: Paint

The Open University — Wed, 01 May 2019 17:18:55 GMT

A particular shade of green is made by mixing 3 litres of blue paint with 5 litres of yellow paint (Figure 23).

How much yellow paint is needed to make this shade of green if you only have 2 litres of blue paint?

Figure _unit4.2.7 Figure 19 Tins of paint

Activity _unit4.2.8 Activity 15 Reflecting

Allow 5 minutes

Look at the learner’s incorrect response below.

How can you explain the thinking behind this approach?

There are 3 litres of blue and 5 litres of yellow paint.
So, there is a total of 8 litres of paint needed.
There are 2 litres of blue paint.
So, to make the 8 litres, 6 litres of yellow paint are needed.

Discussion

This strategy is known as the constant sum strategy. This can be easily tackled using examples for which this strategy is impossible. For instance, if the question had asked ‘How many litres of yellow paint is needed for 12 litres of blue paint?’, the total sum could not have been consistent.

In this particular example, discussions about proportionality and the need for consistency in order to preserve the particular shade of green, may support learners.

Proportional problem 4: Printing press

The Open University — Wed, 01 May 2019 17:18:55 GMT

A printing press takes exactly 12 minutes to print 14 dictionaries.

How many dictionaries can it print in 30 minutes?

Activity _unit4.2.9 Activity 16 Reflecting

Allow 5 minutes

Look at the learner’s incorrect response below.

How can you explain the thinking behind this approach?

The printing press can print 28 dictionaries.

Discussion

This learner appears to have multiplied by 2, despite 12 not being half of 30. This is a common mistake made by learners, who have previously found that doubling has been an effective strategy. This is known as the magic doubling method, when learners use a doubling strategy but it is inappropriate to do so.

Similarly, learners often make mistakes by using multiplication but not by the correct factor.

It is important to offer learners a wide range of varied examples and to have discussions about strategies, identifying that some approaches will work for particular problems but not for others.

As mentioned in the first part of this week, teaching learners methods for solving ratio problems is not enough to develop their proportional reasoning. Learners need to be able to make decisions about when to use different strategies.

Proportional problem 5: Bookshop

The Open University — Wed, 01 May 2019 17:18:55 GMT

Martha buys 4 books for £11. In this bookshop, all books are the same price.

Billy buys 9 books from the same shop. How much do they cost in total?

Activity _unit4.2.10 Activity 17 Reflecting

Allow 5 minutes

Look at the learner’s incorrect response below.

How can you explain the thinking behind this approach?

Table _unit4.2.1

Books	Price
4	£11
8	£22
9	£23

Discussion

This learner has used an incomplete application of the building-up method. They have attempted to use a building-up method, by doubling the original amount to get the price for 8 books and then they have made a mistake in the final step.

In the left-hand column, the learner doubled the number of books and then added a quarter of the original number of books. On the right-hand side, this has been translated to ‘double the price and add 1’.

As the teacher, unpicking this mistake means you can see that there is some understanding to build upon.

Bookshop

Table _unit4.2.2

Books	Price
4	£11
8	£22
1	£2.75
9	£24.75

Adding the extra step into the working could help to avoid the mistake made above.

2.3 Teaching to address common misconceptions

The Open University — Wed, 01 May 2019 17:18:55 GMT

In a topic such as this, there are many common misconceptions. It is important not to avoid these issues but instead to build opportunities to discuss these misunderstandings as part of your teaching. Through identifying mistakes and discussing why answers are incorrect, learners will develop both their proportional reasoning and their problem-solving skills.

3 Connecting ratio and proportional reasoning

The Open University — Wed, 01 May 2019 17:18:55 GMT

According to the Oxford English Dictionary (OED) Online, ratio is defined as ‘the quantitative relation between two amounts showing the number of times one value contains or is contained within the other’.

For example, the ratio of male to female professors is 4 to 1.

In contrast, proportion is described by the OED as a part, share, or number considered in comparative relation to a whole.

For example, the proportion of greenhouse gases in the atmosphere is rising.

In other words, a ratio is used to describe a comparison between two (or more) objects, amounts or values, and a proportion is used to describe one object, amount or value, using a comparison to a whole.

For example if the ratio of boys to girls in the class is 2 to 3, or 2:3, the proportion of boys is 2/5.

Since the proportion of boys in the class is expressed as a fraction of a whole, it is not necessary to know the proportion of girls. If there are 30 children in the class and 2/5 of them are boys, it can be inferred that 3/5, or 18, of them are girls.

In contrast, when expressed as a ratio, the ‘2’ is meaningless without the comparison to the ‘3’.

Proportional thinking and reasoning is knowing the multiplicative relationship between the base ratio and the proportional situation to which it is applied.

Understanding ratio and proportion is more than merely being able to perform appropriate calculations, being able to apply rules and formulae or manipulating numbers and symbols in proportion equations.

3.1 Using simple ratios to make comparisons

The Open University — Wed, 01 May 2019 17:18:55 GMT

First, look at Figures 20–22 and then do the activity based on them.

Figure _unit4.3.1 Figure 20 Using Cuisenaire rods to represent ratios

Figure _unit4.3.2 Figure 21 Using Cuisenaire rods to compare the ratio of lengths

Figure _unit4.3.3 Figure 22 Using multilink cubes to represent ratios

Activity _unit4.3.1 Activity 18 Writing ratios

Allow 5 minutes

Write the ratios for each of the images shown above in the simplest form.

a.What is the ratio of red rods to yellow rods?
b.What is the ratio of the lengths of the green and white rods?
c.What is the ratio of orange to white cubes?

Discussion

8:12 which simplifies to 2:3.
10:4 which simplifies to 5:2.
1:3.

Using manipulatives

The Open University — Wed, 01 May 2019 17:18:55 GMT

When first introducing ratio and proportion, it is important to have a range of concrete resources and manipulative materials available for learners to use. For example, multilink cubes, counters, coins and Cuisenaire rods. (Figure 23)

Figure _unit4.3.4 Figure 23 Concrete, manipulative resources

Cuisenaire rods are a great resource for comparing ratios as the rods can all be related in size. For example, the red rod is twice as long as the white rod, so if the white rod is assigned the value of 3 for a particular question, the red rod will represent 6.

Rods are a great tool for exploring multiplicative relationships.

Note: learners need to be given some time to familiarise themselves with the rods and the relationships between the different rods before they can be useful in the classroom.

If you do not have access to a set of Cuisenaire rods, there is a virtual version of them available on the Nrich website, although this does not provide the same concrete resource that the physical rods do.

Figure _unit4.3.5 Figure 24 Edible resources

Depending on the age of the learners and the teaching setting, using edible resources for work on ratio and proportion can be a concrete resource while providing a meaningful real-life context for learners (Figure 24). The ability to cut up certain foods, such as cakes or pizzas, into halves, quarters and so on, means that fractional ratios can be explored in a practical, hands-on way.

3.2 Connecting ratio and proportion

The Open University — Wed, 01 May 2019 17:18:55 GMT

Throughout this week you have looked at various applications of proportional reasoning. Finally, you will look at how notation for ratio and proportion are linked.

Recapping what was discussed at the start of this section, a ratio represents a relationship between two quantities. A proportion represents one quantity in relation to the whole.

For example, a tropical drink uses 150 ml of orange juice and 100 ml of mango juice.

The ratio of orange to mango, in the simplest form, is 3:2, as for every 3 parts of orange juice, 2 parts of mango juice is needed.

Expressed as a fraction, the proportion of orange juice in the drink is

as there are 5 parts in total and 3 of these are orange juice.

Activity _unit4.3.2 Activity 19 Matching ratio and proportion

Allow 10 minutes

For each of the sets of counters displayed, write the ratio and proportion in the box below.

Interactive content appears here. Please visit the website to use it

Answer

Ratio of red to blue is 3:1

The proportion of red is

Interactive content appears here. Please visit the website to use it

Answer

Ratio of red to blue is 1:2

The proportion of red is

Interactive content appears here. Please visit the website to use it

Answer

Ratio of red to blue is 4:5

The proportion of red is

Interactive content appears here. Please visit the website to use it

Answer

Ratio of red to blue is 2:3

The proportion of red is

Interactive content appears here. Please visit the website to use it

Answer

Ratio of red to blue is 1:1

The proportion of red is

Skip transcript: Video 1 Expressions and terms

Interactive content appears here. Please visit the website to use it

Answer

Ratio of red to blue is 1:3

The proportion of red is

3.3 Using ratio tables

The Open University — Wed, 01 May 2019 17:18:55 GMT

One way to support learners in developing their own mental strategies for solving proportion problems is through the use of ratio tables.

Ratio tables are a way to symbolise the problem and can support learners in finding strategies for solution. They encourage approaches such as halving, doubling, and multiplying by 10.

Activity _unit4.3.3 Activity 20 Reflecting

Allow 10 minutes

Two learners have used ratio tables to work on the problem shown below. They have both taken slightly different approaches.

Have a look at their workings below. Can you identify each of their strategies?

Problem: Seedling plants come in boxes of 35 plants. How many plants would be in 16 boxes?

Table _unit4.3.1 Seedling plants – Sophie’s strategy

Boxes	1	2	4	8	16
Plants	35	70	140	280	560

Table _unit4.3.2 Seedling plants – Alejandro’s strategy

Boxes	1	10	2	6	16
Plants	35	350	70	210	560

Discussion

Sophie’s strategy

Repeated doubling to get to 16 boxes.

This works because 16 is a power of 2. If the question was asking for 15 boxes, she could use the same strategy but then subtract one lot of 35.

Alejandro’s strategy

Multiply by 10.
Separately also multiply original amount by 2, then multiply by 3 to get 6 lots.
Add 10 lots and 6 lots to get 16.

Alejandro has used a building-up strategy. He did not just use a scalar multiplier. Instead, he had to find different parts and sum these. This can be known as the addition and scaling method.

In the next example, Alejandro was given the unit amount (that 1 box contained 35 seedlings). This meant that he only needed to use multiplication.

In the second example below, the unit cost is not given, so some division, as well as multiplication, is required.

Activity _unit4.3.4 Activity 21 Proportion problem

Allow 10 minutes

Think about how you might approach this problem before reading the learner’s response below.

Problem: Mangoes are 2 for £3. How many could you buy for £7.50?

Table _unit4.3.3 Mangoes – Yasmin’s strategy

Mangoes	2	4	1	5
Price	£3	£6	£1.50	£7.50

Discussion

This problem is slightly different from the previous examples because it is not as clear how to get from £3 to £7.50 as it was to get from 1 to 16. Because £7.50 is not a multiple of 3, it cannot be made using either Sophie’s or Alejandro’s strategy.

Yasmin’s strategy:

Double to get £6.
Need £1.50 more so adding another £3 will be too much.
Half the original amount to get £1.50.
Add £1.50 to £6 (1 lot to 4 lots) to get £7.50.

3.4 Continuing to develop learners’ proportional reasoning

The Open University — Wed, 01 May 2019 17:18:55 GMT

This week discussed how learners might approach different problems relating to proportion and ratio. You have considered how misconceptions could be addressed and the types of resources that might help learners to develop their understanding.

Ratio and proportion underpin many key mathematical topics and have applications in STEM subjects as well as real-life contexts. It is important that learners are given ample opportunity to practise, problem solve and discuss proportional reasoning.

4 This week’s quiz

The Open University — Wed, 01 May 2019 17:18:55 GMT

Check what you’ve learned this week by doing the end-of-week quiz.

Week 3 practice quiz

Open the quiz in a new tab or window and come back here when you’ve finished

5 Summary of Week 3

The Open University — Wed, 01 May 2019 17:18:55 GMT

In Week 3 the focus has been on proportional reasoning and its link to multiplicative relationships. You have seen how the use of concrete objects can support early understanding of proportional reasoning, progressing to pictorial representations and, later, abstract mathematical notation as learners’ thinking becomes more sophisticated.

You have worked on a range of proportional reasoning problems, looked at examples of common misconceptions and considered how these could be addressed.

You should now be able to:

define what is meant by proportional reasoning
recognise when a learner is using additive reasoning or when they are using multiplicative reasoning
use effective approaches for teaching proportional representations
have an awareness of the common misconceptions which learners meet when working through proportional reasoning problems.

Next week you will study the first of two weeks on algebra, looking in particular about how the foundation of algebraic understanding is introduced and developed.

You can now go to Week 4.

1 Mathematical sentences and algebraic expressions

The Open University — Wed, 01 May 2019 17:18:55 GMT

It is tempting to define algebra as the strand of mathematics that uses letters for numbers. However, the roots of algebraic thinking lie in paying attention to quantities and their structural relationships. One definition that respects this broader view holds that algebraic thinking can be can characterised by:

Indeterminacy: the problem involves unknown quantities.
Denotation: the unknown quantities are represented, for example by words or symbols.
Analyticity: the unknown quantities are operated on as if they were known numbers (Radford, 2014).

In the next section you will be asked to have a go at an algebraic task. At the end of the section you will consider how it meets these three requirements.

1.1 Writing algebraic expressions

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will write some algebraic expressions which arise from a simple problem.

Activity _unit5.1.1 Activity 1 Triangle perimeters

Allow 10 minutes

The shape in Figure 1 is made of two congruent triangles. Each triangle has sides labelled 3 cm, a cm and b cm. Find an expression, in terms of a and b, for the perimeter of the combined shape. You do not need to simplify it.

Find another expression. And then find another.

Figure _unit5.1.1 Figure 1 Triangle perimeters

The purpose of this task is to work with unknown quantities – the side lengths a and b and the perimeter – which can be represented in different ways: as (labelled) parts of the diagram, in words and in symbolic expressions. A good way to start is by reminding yourself what perimeter means, and by labelling the side lengths of the green triangle (which one is 3 cm long and which is a cm?).

There are different methods for calculating a perimeter, and each of these could give rise to a different symbolic expression. This is the reason for asking you for ‘another expression. And then another’. People working on this task in a group are likely to find a range of symbolic expressions but if they simplify these, they should all come to the same result.

For many learners, handling the congruent triangles will help them perceive how the combined shape is made, which sides are the same length and how to express the ‘leftover bit’ as a difference or a subtraction. We have avoided using right-angled triangles (so as not to remind you of Pythagoras’s Theorem) but this is not a distraction for younger learners. They can cut a rectangle in half diagonally, label the shortest side 3 units, and rearrange the pieces.

Discussion

Teachers have found that a key feature of encouraging early algebraic thinking is productive lingering. This is a phrase to describe asking learners to compare and explain their answers when they are still in the form of expressions that show the thinking processes and the mathematical operations in a calculation, and before they are tidied into a final answer.

Learners can express their method in words, or as a calculation using numbers, or, eventually, as an algebraic expression. These word- or number-based expressions can be called mathematical sentences, and they are the foundations of algebraic thinking.

In Japanese classrooms, teachers have a special word for the expression that leads to a final answer. It is called the ‘shiki’. Teachers ask their learners to work out what is the appropriate shiki to use for a problem, and ask them to say the shiki as well as the final answer.

Activity _unit5.1.2 Activity 2 Reflecting

Allow 10 minutes

How many expressions did you find for the perimeter?

In the table below, match the algebraic expressions with the corresponding explanation in words.

Table _unit5.1.1 Match the algebraic expressions 1–5 with the ‘mathematical sentences’ A–E.

1	P = (a + b + 3) – 3 + (a + b + 3) – 3	A	On the green triangle it’s only a and b because the 3 is missing. On that one it’s 3 and b and a but then you’ve got to take 3 away again.
2	(a + b) + 3 + b+ a – 3	B	The perimeter is 3 add side b then it’s the little bit down there, then it’s add side b and side a. That bit is what’s left over when you put the 3 next to a.
3	P = 3 + b + (a – 3) + b + a	C	To make the perimeter – for each whole triangle it’s a plus b plus 3, but it’s missing 3 from each one.
4	(a + b + 3) + b + (a – 3)	D	Count all the sides, it’s a plus b plus 3, doubled, and then you miss out 3 twice.
5	2 (a + b + 3) – 2 x 3	E	That side is the same as the missing side so you have all of one triangle. For the other triangle you have side b and just a bit down side a, a less 3.

Discussion

All of the expressions are equivalent to 2a + 2b but they represent different thinking processes.

You should have matched 1C, 2A, 3B, 4E, 5D.

The next section examines the idea that you can express mathematical reasoning in words or in algebraic expressions. The mathematical reasoning that you are doing in this task is giving instructions for calculations without actually doing the calculation.

1.2 Expressions and terms

The Open University — Wed, 01 May 2019 17:18:55 GMT

An algebraic expression is a calculation, written in standard arithmetic notation, where one or more numbers has been replaced with a symbol. If an expression involves addition or subtraction then the parts that are added or subtracted are called ‘terms’. The video below shows the parts of an equation which are algebraic expressions and then shows how the expressions are made up of algebraic terms.

Download this video clip.Video player: Video 1 Expressions and terms >

Video _unit5.1.1 Video 1 Expressions and terms

Transcript: Video 1 Expressions and terms

INSTRUCTOR:

An algebraic expression is just a calculation, but one or more of the letters has been replaced with a symbol. Here we've got an equation that comes from the perimeters problem. P, the perimeter, is found by calculating the sum of the five side lengths-- 3, b, the short side length, a minus 3, b, and a. So our equation says that the perimeter is equal to that sum.

The equation is made out of two expressions. One of them is just a single P for perimeter. And that is equal in value to another expression, which is 3 plus b plus a minus 3 plus b plus a.

If an expression involves addition or subtraction, then the parts that are added or subtracted are called terms. So this expression has five terms. We have 3. We have b. Here we have a more complex term, a minus 3. Then we have b, and we have a.

a minus 3 is written in brackets because we calculated the length of that short side by subtracting 3 from a. But it could be separated into a plus a and a minus 3, which would give us six terms in total. So this is to remind you that an equation is made by saying that two expressions are equal, and the expressions can further be broken down into terms.

Here we look at collecting like terms in an expression. The first step looks at what we've already done. Adding a minus 3 is the same as adding a and then subtracting 3. So our expression now has six terms.

Then we collect together the terms that can be added up or subtracted, the like terms, 3 and here subtract 3. These are both just numbers. So I collect those first. There's an invisible plus at the beginning of the first term. So I have plus 3 then minus 3 that can go together.

Then you'll notice we've got a plus b, another plus b, a plus a, and another plus a. What I've done there is to collect those terms together, bringing the operator sign, the plus or minus, with each term. Now I'm going to make that a shorter expression by collecting the like terms.

Plus 3 minus 3 gives me 0. Plus b plus another b, whatever number b is-- and, of course, b could be any number-- a shorter way of writing that is to say that I'm adding 2 times b. And, similarly, plus a plus another a-- a shorter way of adding two lots of a is to write 2 times a. So we have 0 plus 2 times b plus 2 times a.

Finally, we'll use the convention that, instead of writing 2 times b, we write 2b. And, instead of 2 times a, we write 2a. The final expression is 2b plus 2a. If you look at all the different expressions you found for the perimeter, they will all simplify to 2b plus 2a in their shortest form.

End transcript: Video 1 Expressions and terms

Show transcript|Hide transcript

Skip transcript: Video 2 Discussing the misconceptions behind wrong answers

Video _unit5.1.1 Video 1 Expressions and terms

Interactive feature not available in single page view (see it in standard view).

Differences between talking in mathematical sentences and writing algebraic expressions

It is useful for teachers to compare spoken explanations and algebraic expressions.

The explanations in Activity 2 above are written in informal language, not in whole sentences, but as learners might say them.

Learners typically explain ideas in several short sentences, each with one operation. The sentences may follow the order of their thinking (for example, the first instruction will involve ‘adding’ since the aim is a perimeter). This is unlikely to match the order of the terms in the conventional expression.

When we read, we read from left to right. When we talk, our sentences usually start with the main subject. However, in numerical or algebraic expressions, the first term is not the most important one, nor is it always the unknown one, as in the examples above. The order in which you do operations is not necessarily the same order as you read and write the symbols.

Note how learners typically use ‘pointing’ words such as ‘this’, ‘that’ or ‘there’ to indicate a quantity that they want to refer to, but cannot name it without losing the thread of their thinking. In the classroom, they may also point to the diagram or one of the sides while they are talking. Gestures can be an important way of signalling their awareness of an unknown quantity. In an expression, this quantity might appear in brackets, or as a more complex term.

These are steps towards algebraic thinking. Learners have paid attention to an unknown quantity and are communicating about it.

How would you as a teacher build on the learner’s talk, and rephrase each explanation to make a more precise mathematical sentence? Think about using words such as ‘difference’ and ‘less than’.

(We say more precise here since it is not always easy to express mathematical relationships concisely and precisely in words – that is why algebra was invented.)

Activity _unit5.1.3 Activity 3 Reflecting

Allow 5 minutes

Look back in the diagram in Activity 2, how would the task be different if a and b were replaced by specific numbers? How could you work with learners to keep the focus on mathematical sentences?

Discussion

When given specific numbers, learners tend to work towards closed answers consisting of a single term. For example, if the triangles were labelled with sides 3, 5 (for a) and 7 (for b), they would add up as they went around the perimeter and get the answer 24. You would need to ask learners to write down mathematical sentences to keep the focus on the instructions, not the answer, such as writing down 10 + (5 – 3) + 5 + 7 = 24.

When there are specific numbers, learners find it easier to identify a number that they can use for the ‘leftover bit’.

For a general side a, the difference is written a – 3. It cannot be simplified further but you can write it instead of a number and give the instruction to add it to the other sides. Learners often like to use brackets to show that this is an expression that is being treated as a single object.

One strategy for helping those who are stuck on this part is to say ‘Suppose a was 5, how long is that bit?’ There is then a danger that they spot that the difference is 2 using only their knowledge of the specific numbers 5 and 3. You can help them pay attention to the general structure by recapping that the difference is 2 because 5 – 3 is 2, so the important operation is subtraction.

Activity _unit5.1.4 Activity 4 Varying the problem

Allow 10 minutes

What happens to the perimeter if you combine the two congruent triangles into the new shapes in Figure 2?

Use the mathematical extensions for greater depth:

What other shapes and perimeters can you find with these triangles?

Could you use these triangles to make a shape with perimeter 2a + 2b + 3?

Figure _unit5.1.2 Figure 2 Four triangles

Discussion

If you join the two triangles together with the whole side of one triangle set against the same length side of the other triangle, you can arrange the triangles into many shapes, including a rectangle, a parallelogram and a kite. Whatever the shape, the perimeter will always simplify to an expression that is double the sum of two of the sides.

If you change the rule slightly, then you can find some other perimeters. For example, they could meet just at one point, or the sides could overlap by only half their length.

Activity _unit5.1.5 Activity 5 Is this task algebraic?

Allow 5 minutes

The beginning of this section defined algebraic thinking as involving indeterminacy, denotation and analyticity. In other words, it involves thinking about indeterminate quantities that are represented in words or symbols and then treated analytically.

Do you consider the triangle task algebraic?

Discussion

Although the task is set in a geometric context and needs an idea of perimeter, it is algebraic.

It involves indeterminacy, as learners are working with the unknown quantities a, b and p, and these are treated analytically since they are operated on as if they were known numbers. The quantities are represented on a diagram to give a concrete basis for the reasoning. Learners also represent, or denote, them symbolically using letters and numbers and iconically using words to identify and name the quantities (‘the difference’, ‘the little bit’) or to ‘point’ to them (‘that one’). The fact that the answer cannot be reduced to one specific number or closed answer is also characteristic of algebraic thinking.

2 Finding the general in the particular

The Open University — Wed, 01 May 2019 17:18:55 GMT

Early algebra involves moving between the particular and the general.

When we talk about moving to the general in mathematics (also called generalising), we are concerned with finding features of a situation, thinking about how they may change and extending our reasoning to cover those changes. This has many similarities with our aims for learning – we want our learners to generalise beyond today’s lesson.

You have seen that algebraic thinking involves working with unknown quantities, so you can think of this movement in two ways.

First, teachers and learners can choose different representations for the unknown quantities. Some representations are closely linked to their own particular classroom experiences, while others draw on generally accepted conventions for symbolic notation.

Second, the indeterminate quantity being represented can be thought of as having a specific (but unknown) value or it can be thought of as a variable, capable of taking a range of values. Mathematical statements about a specific unknown are likely to lead to finding its value; whereas statements about a variable say something more general, since they must hold for any of its possible values.

This section looks at these two movements, starting with representations.

2.1 Representations

The Open University — Wed, 01 May 2019 17:18:55 GMT

The work of Jerome Bruner (1966) has been influential in early algebra. He identified three modes of representation for mathematical objects: the enactive, the iconic and the symbolic, which move broadly from the concrete to the abstract. Learning a new concept is supported by meeting each of these modes and translating between them.

In the enactive mode, the concept is represented through learners acting on concrete, physical objects. For algebra, the concept of an unknown quantity can be represented by a bag or tin containing an unknown number of counters. The teacher can ask questions such as 'What happens if three counters are placed in a tin? What if one counter is taken out? What if there are ten tins with the same number of counters in each?' A variable can be represented as a changing length, for example learners can measure the height of a sunflower.
In the iconic mode, a picture illustrates the unknown quantity (Figure 3). The picture takes its meaning from the learner’s previous enactive experiences.
In the symbolic mode, the concept is represented in an abstract or a conventional way. Bruner considers words (such as ‘five’, ‘add’ or ‘tin’) to be symbolic in the same way as conventional letters and signs are (e.g. 5, + or t).

Figure _unit5.2.1 Figure 3 Representing operations on an unknown quantity iconically

Concrete–pictorial–abstract

Some teaching traditions describe a learning progression through concrete–pictorial–abstract and develop their materials accordingly. There are often intermediate stages in the final translation towards symbols/abstract representations. For example, Figure 3 shows two different pictorial/iconic representations of the tin activity. One use photographs of objects and arrows that represent the actions of adding and taking away counters. In the other, the learner has drawn their picture of a tin and combined it with familiar symbols such as +, 3 and =.

Bruner’s three modes offer a useful way of planning for progression, but their order does not have to be interpreted too strictly. Teachers have found that even young children can reason algebraically about a quantity that they have not handled, when the meaning is carefully established through discussion. Equally, what feels concrete or abstract to a learner can change with experience. Words such as the ‘price of a book’ are considered as symbolic for younger learners, but they can feel iconic once their language has developed. For example, learners often choose to use symbols such as b (book) or p (price) which remind them of the words they are using for a quantity. A learner studying A-level mathematics will be able to use x, y or any other letter for the price and will eventually have enough experience of manipulating symbols with pencil and paper that this will start to feel like a concrete activity on which she can base further learning.

Making sense of symbols

Research in early algebra suggests that learners need to establish meaning in two ways. First, they need to understand how calculations and arithmetical relationships between quantities are expressed using the familiar symbols for numbers and operations. This is why teachers emphasise forming mathematical sentences in words and in number symbols as well as calculating answers. Second, learners need to make sense of working with an algebraic symbol to represent an indeterminate quantity. These are both necessary stages, which lay the foundations for manipulating symbols out of a context. If learners move too quickly to symbolic representation they risk interpreting the symbols incorrectly, introducing misconceptions that use their prior experiences from number work to deal with the troublesome letter.

Activity _unit5.2.1 Activity 6 Reflecting

Allow 10 minutes

Look at this question and the range of responses. Which would you expect to see from your learners (try it with them)? What is the misconception that underlies each answer?

When you have finished the task watch the video.

Download this video clip.Video player: Video 2 Discussing the misconceptions behind wrong answers >

Video _unit5.2.1 Video 2 Discussing the misconceptions behind wrong answers

Transcript: Video 2 Discussing the misconceptions behind wrong answers

INSTRUCTOR:

We are interested in learner responses to the question-- if a is 4, what is 2a plus 1? We know that 2a plus 1 means 2 times a, add 1.

The most common learner response is likely to be 9, in which case, they're working out 2 times 4-- 4 instead of a-- then plus 1. And the answer is 9. And that's the most common response.

But you're also likely to get a range of incorrect responses. One of these is the answer 3a. In this case, the learner's probably looking at 2a plus 1 and ignoring the letter. So they see 2a plus 1 and write the answer 3. In some cases, they then add the a back in, again, just to do something with it. So that's likely to be an incorrect response you'd get, and we call it letter not used or letter ignored.

This is also the case with the incorrect response 7. Here, we have 2. The a is replaced by 4. And we have plus 1. That's read as an instruction to add up the numbers 2, 4, and 1, which gives 7. Sometimes learners, aware that this is algebra, will add an a in. Those three answers are the ones that you're most likely to find.

Sometimes you'll also find an answer such as 25. Here, the learner looks at the 2a next to each other and thinks about this as a two-digit number. So 2 with a 4 is 24, giving the answer 24 plus 1 is 25. This is quite a reasonable view, as it is how we write two-digit numbers. But in algebra, 2 followed by a has to be recognised as 2 times a.

Finally, you might find you get strange numbers, such as 17. What probably happened here is a confusion between 2a and a squared. 2a is a plus a or 2 times a. But confusing the addition and the multiplication can give us a times a. When a equals 4, that is 4 times 4 plus 1 equals 17.

So those were a range of correct and incorrect responses to the question-- if a is 4, what is 2a plus 1?

You have the correct response-- 9. You have responses such as 3a and 7, where the letter is ignored. And the calculation is done on the numbers available, perhaps with the letter added in afterwards. And you have a sensible, but incorrect interpretation of 2a as a two-digit number.

In your class of, say, 30 learners. Most will give the correct answer. But you will have a few who show these misconceptions. If you see these responses, you will know that they are based on some reasoning, but it includes a misunderstanding of algebraic conventions.

End transcript: Video 2 Discussing the misconceptions behind wrong answers

Show transcript|Hide transcript

Skip transcript: Video 3 Missing number problems

Video _unit5.2.1 Video 2 Discussing the misconceptions behind wrong answers

Interactive feature not available in single page view (see it in standard view).

2.2 Indeterminacy: moving from specific unknowns to variables

The Open University — Wed, 01 May 2019 17:18:55 GMT

When learners are introduced to symbolic representations, they often represent a quantity that has a specific but unknown value. The learner may be asked to find this value.

Two examples of such problems are missing number problems and Think of a Number (THOAN) problems.

Activity _unit5.2.2 Activity 7 Missing number problems

Allow 10 minutes

Have a go at the following problems.

Figure _unit5.2.2 Figure 4 Missing number problems

Activity _unit5.2.3 Activity 8 Reflecting

Allow 5 minutes

What makes these algebra problems, rather than simply number problems?

What do you expect learners to learn from these problems?

Now watch the two videos that discuss these questions.

Download this video clip.Video player: Video 3 Missing number problems >

Video _unit5.2.2 Video 3 Missing number problems

Transcript: Video 3 Missing number problems

INSTRUCTOR:

In this activity, you were asked to think about what makes these algebra problems, rather than number problems, and what you expected learners to learn. Looking at the missing number problems, you can see that they involve mathematical sentences. They're similar to algebraic expressions, as they contain several terms, and they're not closed expressions.

One missing number problem on its own may involve very little algebra. For example, looking at the first one, a learner may think, question mark plus 3 equals 13-- oh, well, I know that 10 plus 3 equals 13. So the answer is 10. There's no algebra involved here. This is using a known number fact instead.

However, if you move to the second question, question mark plus 2 plus 3 equals 13, this could be done in a similar way. Or it could be done by comparison. The question mark plus 2 takes the place of the question mark in the first equation. So a question mark plus 2 is the same as 10. Question mark is 8. It's 8 plus 2 plus 3 that makes 13.

There's something to notice here about algebraic conventions. In each of these questions, in each of these equations, the question mark stands for a different value. That's quite normal in textbooks and in mathematical exercises. But when you start using algebra to model complex situations or problems, we tend to use the same letter for the same variable all the way through while working so as not to get confused.

What do we expect pupils to learn from this? Well, appreciating that you can rearrange and simplify expressions that contain unknown quantities. They could appreciate that they could compare expressions with unknown quantities. For example, we've seen a valid reasoning that the question mark in the second line is 2 less than the question mark in the first line. And they may also appreciate that the equals sign means equal in value rather than makes.

Let's look at that a bit more closely. In each set of problems, they're scoped for the learner to pay attention to generality, perhaps asking, what is the same and what is different in each set of questions. In this first set, we draw attention to the possibility of adding more than two numbers. What's the same is the answer's always 13. What's different is how many numbers are added up to make 13 and also, where the missing number appears in the different positions.

So that's how we have variation in the first set of problems. In the second set of problems, what's the same is they're all equations. And the left-hand side, 10 plus 3, is always the same. However, after the equals sign, we also have expressions that involve unknowns. And that's quite unusual for learners.

Learners expect the equal sign to give an instruction to say makes. And therefore, they expect a single term, an answer, on the right-hand side of the equals sign. So what might a learner do? They might look at 10 plus 3 equals question mark and write 10 plus 3 equals 13 and then carry on, plus 2, and give the answer 15-- question mark equals 15 as the answer there.

This would indicate an understanding of the equals sign as makes. What we're wanting them to do is understand the equals sign as equal in value. So 10 plus 3 is the same in value as 11 plus 2 since they're both equal to 13.

End transcript: Video 3 Missing number problems

Show transcript|Hide transcript

Skip transcript: Video 4 THOAN

Video _unit5.2.2 Video 3 Missing number problems

Interactive feature not available in single page view (see it in standard view).

Download this video clip.Video player: Video 4 THOAN >

Video _unit5.2.3 Video 4 THOAN

Transcript: Video 4 THOAN

INSTRUCTOR:

There are three versions of the think of a number problem. The second version is probably the easiest. The answer is given here. So the reasoning is to write or say a mathematical expression that represents the idea of thinking of a number, multiplying by 3, then adding 2.

So for example, we can take the idea of question mark times 3, add 2 equals 17. This could be solved simply by recalling number facts. I might know it's 5, which if you times it by 3 and add 2, it gives you 17. So the algebra is in writing the expression.

But there's also an algebraic way of solving it by focusing on undoing the sequence of operations on the number. When we do the operations, first, we times question mark by 3, then we add 2. When we undo, we start with 17, and we take them and reverse order. 17 subtract 2 is 15.

Then we undo the times 3 with a divide by 3. 15 divided by 3 is 5. So the number that Harry started with was 5. This version of the problem could be solved numerically with number facts or algebraically. And in this version of the problem, there is one specific value for the unknown. It's only 5 that gives us the answer 17.

Now, look at the first version of the problem. Here, you're just asked to think of a number and then to tell me your answer. This works quite well with the teacher asking somebody from the class and then being very quickly able to tell that person the number they first thought of. It can then be used with partners writing down and hiding their original numbers, telling each other their answers, and aiming to discover their partner's number.

Asking how many of these they can find in five minutes adds some competition to the activity. In this version, the unknown takes on a range of values, because it can be whatever the learners think of. So the algebra comes from comparing the methods used to work forwards, multiplied by 3, then adding 2, to comparing the methods used to work backwards. That's subtracting 2 and dividing by 3.

Asking the learners to work forwards themselves first gives them the experience from which they can reason about how to work backwards. Finally, in the last question here, a slightly different thing happens. You think of a number, you double it, add 6, divide in half, subtract the number you started with, and the answer is always 3, no matter what number you start with.

Here, we're getting to the idea of an unknown quantity as a variable. So how can I represent that? My number could be n. When I double it, it's 2n. I then add 6. It's 2n plus 6. I divide it in half. It's 2n plus 6 divided by 2. And I subtract the number I started with.

So I have to subtract. And I'm using n again. Subtract n, because that's the number I started with. The answer will always be 3. So I can write that as equals to 3. And the algebraic question is, why is that always true? The algebra here is used in asking learners to explain convincingly why the answer is always 3.

End transcript: Video 4 THOAN

Show transcript|Hide transcript

Video _unit5.2.3 Video 4 THOAN

Interactive feature not available in single page view (see it in standard view).

2.3 Comparing algebraic expressions

The Open University — Wed, 01 May 2019 17:18:55 GMT

Learners find it easier to follow word problems that are posed in a way that matches a typical sentence structure, such as the THOAN problems above. However, even a small variation on these will start to disrupt the order. For example, it is much harder to work on a problem like this one (which comes from an examination for 11-year-olds):

Younus takes three large letters to the Post Office. He pays with a £20 note. He gets change of £14.80. How much did each letter cost to post?

Earlier you have seen there is a progression in moving from working with a quantity that represents a specific unknown to one that can take a range of values. You will return to the ideas of variables and generality from a different perspective in Week 5, when you will look at introducing functions.

In the middle years, learners will move between these perceptions of number as they tackle different problems. Some learners strongly prefer to think of an unknown quantity as having a specific value. They experience what researchers call ‘a compulsion to calculate’. You can test this preference by asking diagnostic question such as these:

Activity _unit5.2.4 Activity 9 Reflecting

Allow 5 minutes

How did you answer these questions?

Did you choose specific values for m and n? Do you see now that you did not need to?

Discussion

The answers are: 11, 51 and 17.

The first question involves such a familiar number bond that you are very likely to have thought of: m as 6.

The second was chosen to have the same structure but vary the number involved so that it becomes easier for you to pay attention to the comparison of 2m and 2m – 1. Some of your learners will not do this. Instead they will work out m = 26 and then calculate .

The third question again has a similar structure but here m and n can take a range of values, so they are not specific unknowns. Again, some learners will choose values, for example 10 and 5, and then work out 10 + 5 + 2 = 17. Some will show awareness that these are not the only possible values by trying another pair, for example 7 and 8.

Another way is to use tasks that allow learners to treat quantities as variables while providing some support by being embedded in a number context.

The strategies discussed above lead learners to a correct answer but they reveal the ‘compulsion to calculate’. These learners have difficulty in dealing with unknowns by comparing algebraic expressions, and this will prevent their progression to more complex algebra.

One way of working on this difficulty is for teachers to ‘productively linger’ with algebraic problems – asking learners to explain their thinking and listen to others. Obtaining the answer is not enough: learners should use two different methods and be able to compare them.

Activity _unit5.2.5 Activity 10 Symbol sums

Allow 10 minutes

This task involves working with several unknowns and there are several different ways to approach it. Have a go at it yourself.

Each symbol in the grid in Figure 5 has a value. The totals of the rows and columns are shown. Can you find what each symbol is worth?

Figure _unit5.2.3 Figure 5 Symbol grid

Activity _unit5.2.6 Activity 11 Thinking points

Allow 5 minutes

Where would you encourage learners to start?
How long would you leave learners to work independently before you ask them to listen to other people’s strategies?
What kind of verbal explanations would you expect from learners? How do you want them to record their thinking?
Is the task finished when the first learner works out the values? How could you ‘productively linger’ over this task to encourage algebraic thinking?

You can find a similar activity at https://nrich.maths.org/ 1053

Discussion

You could start the problem in several ways:

The first row gives you the sum of heart and smiley.

Compare it with the fourth column to find the value of a cloud.

Compare the second and third rows to find that heart is 1 more than smiley.

The fourth row shows a cloud has to be a multiple of 3: try 3 or 6 or 9.

Compare the third and fourth column to see that a diamond is 3 more than a smiley.

The third row and third column show a diamond is 5 more than a lightning.

Activity _unit5.2.7 Activity 12 Symbol sums continued

Allow 5 minutes

Figure _unit5.2.4 Figure 6 More symbol grids

It is a good exercise of your own mathematical knowledge to invent similar problems and check that they can be solved.

In the two grids in Figure 6, the symbols in the grid are the same but the level of mathematics is slightly harder. Can you see why?

3 The equals sign

The Open University — Wed, 01 May 2019 17:18:55 GMT

Figure _unit5.3.1 Figure 7 Friends = love

One of the strong findings of early algebra research is that learners in the middle years need to change their perception of the equals sign. This sign is recognised from an early age (Figure 7), and it is usually understood as standing for an instruction such as ‘makes’ or ‘work it out now’. This is called the operational understanding of the equals sign and it works well for number problems. Text books, worksheets and even national examinations use the equals sign in this way:

To make progress in algebraic thinking, learners need to develop a new relational understanding of the equals sign. In algebra, the equals sign only makes sense when it is used as a way of comparing two expressions and saying that they have the same value as each other.

All of the following equations can be understood as statements that express a relationship between two quantities:

Two other relational symbols are the inequality signs < and >. Tasks that require learners to decide which of these symbols to use can develop a relational understanding of the equals sign.

Activity _unit5.3.1 Activity 13 Relationships between expressions

Allow 5 minutes

Which is bigger: 3a – b or 2a + b? Choose values of a and b and decide. Record your results using the signs <, > and =.

Discussion

You could write:

If we choose then and , so .

If then and so .

Or then and

Activity _unit5.3.2 Activity 14 Reflect

Allow 5 minutes

You might have noticed the different ways in which the three responses to the previous question were written.

How would you ask your learners to record their results?

Does this ensure that they have practice in selecting and writing the correct signs, in comparing two expressions?

Table _unit5.3.1

a	b	3a – b	2a + b	Write your inequality or equation here
1	2	1	4	3a – b < 2a + b
10	3
6	3

Discussion

Using the equals or inequality signs between numbers or symbolic expressions is correct mathematics. Learners should not write =, < or > on its own.

Using a table is often helpful for keeping track of several calculations.

Table _unit5.3.2

a	b	3a – b	2a + b	Write your inequality or equation here
1	2	1	4	3a – b < 2a + b
10	3	27	23	3a – b > 2a + b
3a – b	2a + b	15	15	3a – b > 2a + b

3.1 Relational understandings

The Open University — Wed, 01 May 2019 17:18:55 GMT

Treating the equals sign as a relation is important for two aspects of algebraic thinking. The first is maintaining equal value. The two expressions on either side of the equals sign are equivalent; their values balance. Anything that is done to one side of the equation must be done to the other side, in order to keep that balance. This aspect is seen in the common metaphor of an equation as a balance scale (Figure 8).

Sometimes this aspect is referred to as sameness. However, looking at the balance makes it obvious that there is not the same expression on both sides. Instead, they have equal value.

Figure _unit5.3.2 Figure 8 Balance

What is the equation represented by this balance?

What happens if you take 1 from both sides of this balance? If you take the number represented by x from both sides?

The second benefit of the relational understanding of the equals sign is that it allows substitution. If two expressions are equal in value then one can be substituted for the other. This is useful in simplifying expressions and solving equations.

You met substitution in the symbol grid activities. Watch the video below to see how.

The equals sign as substitution is used twice in this process: the first time it is used to replace an indeterminate quantity (expressed symbolically as a sum) with a number that has equal value. The second time the recall of number facts is used to determine a number that could be substituted for symbolic quantities, and a new equation can be written.

Download this video clip.Video player: Video 5 The equals sign – substitution >

Video _unit5.3.1 Video 5 The equals sign – substitution

Week 4 compulsory badge quiz

Video _unit5.3.1 Video 5 The equals sign – substitution

Interactive feature not available in single page view (see it in standard view).

Substitution or ‘do the same to both sides’

It is useful for teachers to be aware of the substitution aspect of the equals sign as learners will use this in their speech. They are likely to draw on the substitution when they know the number facts involved (here it is 9 + 6 that equals 15). When they do not know the number facts, they will use the principle of ‘maintaining equal value’ and ‘do the same to both sides’ in order to work out 15 – 9 = 6. Both aspects need to be recognised and taught.

3.2 Always, sometimes, never true?

The Open University — Wed, 01 May 2019 17:18:55 GMT

As previously discussed, algebraic expressions are like mathematical phrases or sentences. Equations are like mathematical statements. A mathematician will always ask whether or not an equation is true.

When equations contain no unknown quantities, use calculation to check if they are true:

is an equation but it is not true.

is an equation that is true.

When equations include unknown quantities, they may be true for only one value of the unknown:

is only true when n is 11.

If n = 10 then 2n – 5 = 17 is false.

They may be true for several values:

is true when n = 3 and when n = –3.

They may be true for all values of the unknown:

In formal mathematics you can use the sign ≡ to show that these expressions are ‘identically equal’:

Sometimes an equation is given as a definition or a formula for working something out. We can assume that someone else has done the work of showing it is always true. For example:

(definition).

(formula a learner could prove).

(formula that a learner could not prove!).

Activity _unit5.3.3 Activity 15 Always, sometimes or never true

Allow 10 minutes

Decide whether these equations are true for all values of the variable, or only some values or none.

Figure _unit5.3.3 Figure 9 Always, sometimes, never true?

Discussion

Trying values will help you see that there are no values of b or m that make their equations true: they are never true. Using algebraic reasoning will help us see that e = 1, t = 1.5, c = 0, q = 10 are specific values that make their respective equations true. Also, some equations are true for a range of values: k < 15, p either greater than 2 or less than -2, and x greater than 1 or less than 0. A final one, that is more subtle because it involves two variables, is that p + 12 = f + 12 is true only when p = f. All these equations are sometimes true.

Finally the equations 2n+3 = 3+2n and 16s² = (4s)^₂ are true for all values of n and s. The first is because addition is commutative, and the second because squaring applies to the whole bracket. They are always true.

4 This week’s quiz

The Open University — Wed, 01 May 2019 17:18:55 GMT

It’s now time to complete the Week 4 badge quiz. It’s similar to previous quizzes but this time, instead of answering five questions, there will be 15.

Remember, this quiz counts towards your badge. If you’re not successful the first time, you can attempt the quiz again in 24 hours.

Open the quiz in a new tab or window and come back here when you’ve finished.

5 Summary of Week 4

The Open University — Wed, 01 May 2019 17:18:55 GMT

In Week 4 you met the idea that algebra is more than simply using letters for numbers. Algebraic thinking involves an appreciation of structure and generality. It allows us to work with unknown quantities as if they were numbers, using words or symbols to represent those numbers. You have seen how algebraic thinking can be supported by the progression in the use of concrete, pictorial and abstract representations.

You should now be able to:

describe algebraic expressions and algebraic terms
support learners to move between working with numbers and working with symbols
deal carefully with the equals sign and be aware that it is often used to mean ‘makes the answer of’ but that in algebra it has the meaning of equivalence (i.e. both expressions on the right and left of the equal sign have the same value).

Next week you will build on what you learned in week four to begin to think about functional reasoning and specifically how unknown quantities can vary.

You are now halfway through the course. The Open University would really appreciate your feedback and suggestions for future improvement in our optional end-of-course survey, which you will also have an opportunity to complete at the end of Week 8. Participation will be completely confidential and we will not pass on your details to others.

You can now go to Week 5.

1 Function machines

The Open University — Wed, 01 May 2019 17:18:55 GMT

The function machine is a way of thinking about three aspects that make up a function: the inputs, the outputs and the rule that determines their connection (Figure 2). The representation as a machine connects them in one diagram (just like seeing a toaster makes you think of bread and toast).

You have already seen examples of such thinking in the ‘Think of a Number’ problems. The input is the number you think of, you calculate given a rule and you tell someone the answer – the output.

Figure _unit6.1.1 Figure 2 Function machine

The rules for functions can be as simple as ‘double’ or ‘stay the same’ and as complicated as you like. (It does not even have to be expressed in symbols.) The rule does have to be ‘well defined’, meaning that for each input number there is only one possible answer.

This condition means that two of the examples in Figure 3 are not functions:

Figure _unit6.1.2 Figure 3 Examples and non-examples of functions

As you saw with Think of a Number problems, the function machine representation is powerful because learners can work with it at the level of taking a specific unknown quantity – the input – and calculating with it to get an output. In this sense, the function is a process that they can carry out.

The machine representation also lends itself to more sophisticated tasks requiring learners to go beyond simply finding outputs. Teachers can introduce the idea of the input as a quantity that can take any value. They can set tasks where the process of writing and comparing the rules and outputs is more important than what numbers they are. The two activities you will carry out in this section illustrate this kind of task.

The goal of working in this way is laying the foundations for learners to treat quantities as variables and to use functions fluently in later mathematics.

For example:

the squaring function is used in finding the area of a circle
the cubing function is used in finding volume
the sine function is used in trigonometry (and, of course, the cosine and tangent functions).

The ‘machine’ analogy suits these examples since it corresponds to learners’ experiences of pressing the calculator key for that function (Figure 4). Before calculator technology, all of these functions were either calculated or looked up in special booklets of tables.

Figure _unit6.1.3 Figure 4 Functions on your calculator

Activity _unit6.1.1 Activity 1 Getting used to long expressions

Allow 10 minutes

On a piece of paper, write an expression that summarises the following sequence of functions.

Think of a number, then:

add 3
multiply by 2
subtract 5
divide by 3
add 72
multiply by 6.

Then watch the video below from the OU archives. It introduces a teacher, and shows him building up the sequence with 14-year-old learners on a board. Ask yourself:

What do you think the teacher’s learning objectives are?

How do learners react to calculating on an unknown quantity?

What does the teacher do to draw attention to conventional symbol use?

Download this video clip.Video player: Video 1 Working mathematically with symbols at Key Stage 3 >

Video _unit6.1.1 Video 1 Working mathematically with symbols at Key Stage 3

Skip transcript: Video 1 Working mathematically with symbols at Key Stage 3

Transcript: Video 1 Working mathematically with symbols at Key Stage 3

DAVE HEWITT:

When I first started teaching, I entered into a school where the diet was mainly textbooks or work cards. And I felt that the way the pupils were being asked to work was getting them to do rather repetitive tasks and to always seek an authority somewhere, whether it is me or whether it is the answers at the back of the book.

I was very unhappy with that. And I worked quite hard at working with them in a different way where I tried to find out what they could do themselves. They have minds they bring with them, all sorts of experiences and abilities. And I spent quite a long time, many years, in classrooms finding out what they can do if left, in a sense, to their own devices.

And I found that the pupils realised that they can be quite creative in mathematics and that they are functioning mathematicians at their level. And I saw many pupils grow as a consequence of that. And quite often, I would arrive in a classroom. And if I happened to be a bit late, then quite often, they were working already and they were quite autonomous in that sense.

I then also began to ask the question, well, what is my role? And I felt that I wanted to see whether I could possibly accelerate this learning so that they might be able to learn faster just as comfortably. And as a consequence, I think I've begun working in ways where I'm quite controlling what's happening in the classroom. I'm very much controlling it. And in many ways, the activity relies on me as a controlling body in that room.

I hope, and I feel that I succeed, in still making them realise that they are still in control. So I'm not telling them what to do, but I am trying to affect their attention and perhaps direct their attention to different things, so that they can notice things, perhaps a little bit quicker than if their attention wasn't drawn to certain things.

These lessons are not every lesson. I won't control it in that way every lesson. It's quite often just a starter that may last for one or two or three lessons at the most, I would have thought. What am I going to do to this one? Oh, yes, I'm going to add 3, times by 2, take away 5, divide by 3, add 72.

Got a problem with this? Do you want me to write down what I'm doing?

STUDENTS:

Yes.

DAVE HEWITT:

So-- let me see. I'm thinking of a number. I add 3. Then I'm going to multiply by 2. Then I'm going to take away 5. Then I'm going to divide by 3. Then I'm going to add 72. And then I'm going to multiply by 6. And I get 100. So you're going to-- Jemma.

STUDENT 1:

100 divided by 6.

DAVE HEWITT:

100--

STUDENT 1:

Divide by 6.

DAVE HEWITT:

And you're dividing by 6 because?

STUDENT 1:

Because it was times 6. And so you take that and it's divide by 6.

DAVE HEWITT:

So that's done.

STUDENT 1:

Yeah. Then take 72.

DAVE HEWITT:

OK, and after that-- I've done that.

STUDENT 1:

Times 3.

DAVE HEWITT:

Because I--

STUDENT 1:

Divide 3.

DAVE HEWITT:

So you're going to--

STUDENT 1:

It's times 3.

DAVE HEWITT:

Times by-- all right. We're left with this.

STUDENT 1:

Divide it by 2. Or no-- add 5.

STUDENT 2:

Add 5, divide it by 2.

DAVE HEWITT:

So I add--

STUDENT 1:

DAVE HEWITT:

Right. So that's that done.

STUDENT 1:

Divide by 2.

DAVE HEWITT:

Divide by 2.

STUDENT 1:

Take 3.

DAVE HEWITT:

And take 3. And I end up with-- when I first started off saying I'm thinking a number, what did I write down?

STUDENT 1:

DAVE HEWITT:

So I end up with whatever that number is, and we could work it out and find out what it is. All right.

End transcript: Video 1 Working mathematically with symbols at Key Stage 3

Show transcript|Hide transcript

Skip transcript: Video 2 Different representations of covariance

Video _unit6.1.1 Video 1 Working mathematically with symbols at Key Stage 3

Interactive feature not available in single page view (see it in standard view).

Discussion

The emphasis of the video activity is on gaining familiarity with symbols to express the whole sequence without any intermediate results being calculated. Note how the teacher builds up the expression first by ‘doing’ each function, and then by ‘undoing’ it. ‘Doing and undoing’ is an important idea in algebra. It provides an organising structure to relate processes that appear different but are the reverse of each other, such as × and ÷. This allows us to reduce the complexity of calculations.

The next activity is a well-tested teaching approach devised by Don Steward, called ‘both ways’. Using two functions (× 2 and + 3) increases the level of complexity, and going both ways allows learners to make predictions about the results. This task is designed to shift attention from calculating outputs towards comparing methods of calculation. The activity is written in the way you would use it with a class of learners, although you would need to offer support for some of the calculations (for example, a negative number line).

Activity _unit6.1.2 Activity 2 Both ways

Allow 10 minutes

Figure _unit6.1.4 Figure 5 Both ways

You are going to choose an input number and follow the instructions on the arrows in Figure 5 – both ways. Do you think your answers will be the same?

Try starting with these numbers: 5, 8, 3.1, –4, 100, 1000
a million (you could write m for short)
a billion (b for short)
any number (n for short)
Compare the two answers you get. Can you explain what is happening?

Discussion

You get different answers by going different ways:

13 (green way) and 16 (red way), 19 and 22, 9.2 and 12.2, –5 and –2, 203 and 206, 2m+3 and 2m+6, etc.

The difference between the answers is always 3, for any input number.

Using m for a million, etc. is a popular way of introducing letters for numbers in function machines. It matches the way we speak about such numbers in words: 2 million and 3. It is mathematically correct in that m, b, t all stand for numbers – one million, one billion, etc. – rather than objects such as mangos or bananas that cannot be calculated with.

Although it is a good introduction, it is not an approach to stay with. (For example, you do not want learners to write 2 million one hundred and three as 2m1h+ 3 or think that m always stands for a million.) This is why the sequence of examples ends with using n for ‘any number’ (the sound rhyme helps learners accept this meaning).

You could explain this result with diagrams using a rectangle for n and dots for each 1. The important step is that, following the red arrows, both the number and the 3 dots get multiplied by 2.

This activity shows the need to use brackets when you combine addition and multiplication in a single expression. If you are confident with algebra, you might have explained it to yourself by writing the expression for each way and expanding the brackets. However, learners in middle school are unlikely to be convinced by manipulating symbols because they do not yet feel concrete to them.

green arrows 2n+3

red arrows 2(n+3) = 2 × n + 2 × 3 = 2n + 6

2n and 2n are the same number, but 6 is more than 3 so the difference is 3.

You can find resources related to this activity on the Median blog at https://donsteward.blogspot.com/ indexed under ‘Expressions Both Ways’.

2 Sequences as functions

The Open University — Wed, 01 May 2019 17:18:55 GMT

A sequence is a collection of numbers or objects that are arranged in order. Each of these numbers or objects is called a ‘term’, so a sequence has a first term, second term, third term, etc.

Note that this is the same word, but not the same meaning, as the ‘terms’ that you find in an expression. You do not rearrange, add or subtract the terms of a sequence. It is wise to avoid using the word in different ways within one lesson.

All of the sequences you meet in school mathematics have a rule.

Activity _unit6.2.1 Activity 3 Sequences and rules

Allow 5 minutes

What is the next term in each sequence in Figure 6?

What is the rule for each sequence? Write it in words, and as an algebraic expression if you can.

Figure _unit6.2.1 Figure 6 Sequences

Keep a note of your work on this activity as you will use it in the next section.

Discussion

For the first five sequences the sixth terms are 30, 96, 1/36 and S. The last sequence is a diagram with six triangles.

2.1 Term-to-term and position-to-term rules

The Open University — Wed, 01 May 2019 17:18:55 GMT

For three of the sequences in Figure 6 you probably thought about how to get from one term to the next, and continued that pattern. You could have done this by adding 5, doubling, or drawing another triangle. These are all ways of describing the change between any two consecutive terms, so this is called finding a term-to-term rule.

It would be much more difficult to identify a term-to-term rule for the other two sequences in Figure 6. Instead, you need position-to-term rules. This type of rule connects the position number (1, 2, 3, etc.) to the associated term. A table is a useful representation to show the relationship between position number and the term.

Table _unit6.2.1

Position

one

two

three

four

five

six

Term

(letter sequence)

Term (fraction sequence)

1/4

1/9

1/16

1/25

1/36

1/n²

For the letter sequence, the position-to-term rule is ‘the term is the first letter of the position number as an English word’.

For the fraction sequence, ‘the term is the fraction 1 divided by the position squared’. You can write this symbolically as

Term-to-term rules describe the pattern you see by moving along a row of the table.

Position-to-term rules describe the pattern you see when moving down from the position row.

The rule must describe a general pattern that holds for every term of the sequence.

Learners often prefer looking for term-to-term rules. This is for two reasons. First, they only need to look along the changes in one variable – along the terms of the sequence. Second, term-to-term rules involve simpler operations. However, a term-to-term rule is not helpful if you want to find a term that is far away from the ones that are known.

The position-to-term rule is more powerful.

Activity _unit6.2.2 Activity 4 Position-to-term rules

Allow 10 minutes

Imagine finding the 37th term in each of the five sequences in Figure 6.

Which one would be easiest?

Which one would be hardest?

Write the position-to-term rules for the first two sequences.

Discussion

Table _unit6.2.2

Sequence	37th term	Position-to-term rule
5, 10, 15, 20, 25	185	Multiply position number by 5. n^th term is 5n
3, 6, 12, 24, 48	3 × 2³⁶ = 206 158 430 208; most calculators will not display all of these digits.	Raise 2 to the power of one less than the position number. Multiply the answer by 3. n^th term is 3 x 2^n-1
1, 1/4, 1/9, 1/16, 1/25, ...	1/ 37²	n^th term is the fraction 1 divided by the position squared. n^th term is1/n²
O, T, T, F, F	T	take the first letter of the position number

There is another reason to emphasise position-to-term rules. Sequences are an early example of functions. If we relate this to the function machine: the position number is the input, the term is the output and there is a rule that connects them.

When learners start to work with sequences, they work with specific values of the position numbers: 1, 2, 3, etc. These are not indeterminate quantities, so they are thinking numerically. The algebraic thinking only starts when learners consider what the rule is for a general input number, which is usually denoted by the symbol n. The corresponding term is called the nth term.

It is relatively easy to understand how the position number can change, since it follows the natural numbers. So, although, teachers may never mention the word ‘function’ when teaching sequences, by emphasising position-to-term rules they are nevertheless preparing the underlying idea of a function as a relationship between two variable quantities.

2.2 Generalising with sequences

The Open University — Wed, 01 May 2019 17:18:55 GMT

Sequences also give learners a context in which to generalise, particularly when they are defined using diagrams or shapes. If you look at sequence resources you will find two main approaches to generalising activities. We can describe these as looking for changes versus looking for structure. Many textbooks written in the 2000s focus on the first approach, while more recent research (Kieran et al., 2016) suggests that learners also need to look for structure. The next two activities in this section exemplify each approach.

In the first approach, the intention is for learners to observe a pattern of change across a sequence of examples by asking ‘What is the same and what is different?’

Activity _unit6.2.3 Activity 5 Looking for changes

Allow 10 minutes

Each term of the sequence in Figure 7 is a house. Draw the next two terms.

What is the same and what is different about the houses?

What would the 26th house look like?

Figure _unit6.2.2 Figure 7 A sequence of houses

How many matches are needed for each of the houses? Complete the table below to make a number sequence.

Then describe a method for finding how many matches are needed for house n.

Table _unit6.2.3

Position	1	2	3	4	10	26
Number of matches

Discussion

Did you continue the pattern by looking at the changing shapes?

Or by thinking about numbers – how many matches you would need?

You will explore each of these ways of finding the n^th term. (This is the position-to-term rule.)

Changing shapes

The Open University — Wed, 01 May 2019 17:18:55 GMT

Learners will come up with several different ways of looking at the way these ‘houses’ grow. As you saw in Week 4, having multiple solution methods can give rise to fruitful algebraic reasoning if there is time planned in the lesson for learners to compare their methods, correct errors and explain why they are the same.

One very common method of describing the changing shapes is to focus on the 3 horizontal rows of matches. Another method is to note that each house has the same beginning and the same end. The middle section grows each time by adding 3 matches (1 each to the roof, the floor and the middle). We will now follow this second method to show its details.

The method is described in pictures, and then words and symbols. This sequence can be used for any methods.

Pictures (see Figure 8)

Figure _unit6.2.3 Figure 8 Growing houses

Words

The number of matches for the 10^th shape will be 5 (for the beginning) plus 2 (for the end) plus 3 for each of term added on after the first. You add one less group of 3 than the shape number.

Number symbols

It can be helpful for learners to see the calculations written as a sequence.

Shape 1: 5 + 2 or 7

Shape 2: 5 + 3 + 2 or 7 + 3

Shape 3: 5 + 3 + 3 + 2 or 7 + 3 + 3

Shape 4: 5 + 3 + 3 + 3 + 2 or 7 + 3 + 3 + 3

Now that we have got past 3 × 3, it is worth moving to multiplication instead of repeated addition.

Shape 5: 5 + 4 × 3 + 2 or 7 + 4 x 3

Words and algebraic symbols

Looking at the calculations shows a pattern. A method for finding the nth term is (starting with the part that changes) to calculate the number that is one less than n, multiply that by 3 and add it to 7.

In symbols this is (n-1) × 3 + 7. We can simplify this by expanding the brackets:

3n – 3 + 7

3n + 4.

So, the nth term of the sequence is 3n + 4.

2.3 Changing numbers

The Open University — Wed, 01 May 2019 17:18:55 GMT

If, instead of looking at the shapes, you focused on the number of matches, you probably noticed that the terms went up the same amount each time, in threes. Sequences that have a constant difference have two names (teachers need to know these but learners do not). They are called linear or arithmetic sequences.

For linear sequences, it is possible to work only with the numbers and ignore the shapes, and some teachers prefer to work like this. They rely on learners remembering a procedure:

Remember that the 3 times table increases in threes.
Add a row in the table for the multiples of 3. This row has the rule 3n.
Work out what the change is between 3n and the sequence you want. Adapt the rule to include this change

Let’s demonstrate this procedure.

Step 1 and 2

Table _unit6.2.4

Position	1	2	3	4	10	26
Number of matches	7	10	13	16
3n	3	6	9	12	30	78

Step 3

Looking at the third and the second rows (e.g. look at 3 and 7, or 6 and 10), the difference between them is + 4, so we write the rule as

number of matches = 3n + 4

This helps complete the table for the 10^th and 26^th terms.

Should teachers work with the shapes or the numbers? The advantage of working only with number patterns is speed. It provides a relatively quick method for finding a position-to-term rule for a given linear sequence. On the other hand, it is relatively common for learners to misapply this procedure and give the answer 3n – 4. Another disadvantage is the reduced opportunity for thinking algebraically since learners tend to focus only on the first term of the sequence (so they are not working with indeterminate quantities). In addition, simply finding a position-to-term rule is a skill that is worth a few marks in examinations, but is not used directly to support other topics. If learners can use the numerical method without writing an extra row in the table, it is a good indication of fluent thinking with symbols. If they cannot, the teacher needs to balance the effects on their learners of spending time memorising and perfecting the procedure, or of trying to make sense of change within a context.

2.4 Looking at structure in a sequence

The Open University — Wed, 01 May 2019 17:18:55 GMT

We now return to the second main approach, where the focus is on looking for structure in a sequence. This is similar to looking at shape, rather than number, but it uses only one term from the sequence. A typical term is chosen – usually the third or the fourth, since the first term is often too simple to show the structure.

Activity _unit6.2.4 Activity 6 Looking for structure

Allow 10 minutes

Figure _unit6.2.4 Figure 9 shape C pattern

An activity based on this approach will have two phases of exploration.

In the first short period, learners decide on their ‘shape four’. They can be supported by asking ‘Where do you see the “threes” in shape three?’. This phase ends with collecting several examples of ‘shape fours’, displaying them for comparison and asking ‘If this is shape four, do we know how to make shape five?’

For this problem, you might allow the class to divide into two groups: those who want to extend only the ‘arms’ of the C (so that the fourth shape has 9 squares) and those who have also extended its ‘spine’ (so it has 10). Either method can be justified but they lead to different expressions.

The task for the second phase is for pairs to explain in their books using a labelled diagram what ‘shape one million’ would look like, and find how many squares would be used for any shape.

Isometric dotty paper is a good resource for creating more patterns to practise this approach (Figure 10). Ask ‘If these are shape 3, what is shape 4?’

Figure _unit6.2.5 Figure 10 More shape threes

2.5 Summary

The Open University — Wed, 01 May 2019 17:18:55 GMT

So far this week, you have met the idea of a function machine, with input and output, and the idea that sequences are functions where the input is the position number and the output is the term. In both of these representations, the function can be considered as a process, that is, as something that you do to the input.

In Activity 1 you became more sophisticated in your treatment of the function by reversing the process. In Activity 2 you compared two functions. In Activities 3 to 6 you also saw how a function can be considered as a relationship between two quantities, that is, as something to notice and describe. This relationship stems from the structural connections that link the two quantities (which gives the position-to-term rule) and it determines how they change together. This will be the focus of the next section.

3 Extending functions: variables and graphs

The Open University — Wed, 01 May 2019 17:18:55 GMT

The idea of a function as a relationship is sometimes called covariance; because it describes how two quantities vary together. If your learners go on to study advanced mathematics, they will need to be able to shift fluently between the process perspective on function (associated with rules, function machines and sequences) and the relationship (covariance) perspective.

In the middle years, the goal for most learners is to have a sound understanding of a function as a process and to develop some awareness of the representations related to functions that will prepare them to notice different kinds of covariance.

In this section you will meet some of the more advanced vocabulary and concepts associated with these representations.

3.1 Variables

The Open University — Wed, 01 May 2019 17:18:55 GMT

You have seen that an indeterminate quantity can be thought of as a specific quantity that happens to be unknown (for now), or as one of several unknown quantities. Extending this idea leads to thinking of a quantity as a variable, that is, as a single object that can change, or vary in some way.

Variables can be discrete or continuous.

A discrete variable can take only certain values, and it would be nonsensical to choose a value between them. The position number of a sequence is an example of a discrete variable. A position number can only be a whole number. It would be nonsensical to talk about the ‘3 and a halfth’ term.

A continuous variable is one which can take any value. The nature of a variable may depend on how it is measured. One of the common variables that learners will meet in school is time. Time is usually a continuous variable, but we can make it discrete by taking daily, or hourly, aggregate measurements. You will return to this idea in Week 8 (Working with data).

Activity _unit6.3.1 Activity 7 Representing covariance

Allow 15 minutes

Look at the four pairs of variables below.

Are the variables discrete or continuous?
Informally, how do you visualise each pair of variables changing together?
- Time in hours / Time in minutes
- Age / Number of hours of sleep
- Number of people sharing a box of 24 chocolates / How many chocolates each person gets
- Your age / Your elder sibling’s age.
Now watch the video below which shows different mathematical representations of the first example.

Download this video clip.Video player: Video 2 Different representations of covariance >

Video _unit6.3.1 Video 2 Different representations of covariance

Transcript: Video 2 Different representations of covariance

INSTRUCTOR:

Covariance is an important concept. It describes the relationship between variables that change together. Mathematics has conventional ways of representing covariation, or you might have your own images.

Think about the two variables, time in hours and time in minutes. You might visualise them as the hour hand and the minute hands on a clock. Both measuring time, but with the minute hand moving faster than the hour hand. Or you might think about the variables of specific number equivalences. One hour is the same as 60 minutes. Two hours are 120 minutes. And so on.

This pattern gives us a rule. The number of hours, multiplied by 60, is equal to the number of minutes. The function machine is the closest mathematical representation of this rule. For a specific number of hours, I can calculate a specific number of minutes.

Another way of thinking about the two variables is to think of them as two lengths that change together. This representation is called a double number line. It shows the comparative size of each variable. Both times start at zero and they both increase. But the time in minutes grows 60 times as fast.

We're more used to seeing this comparison of two variables in the two dimensions of a graph. One variable is measured on the x-axis. The other is measured on the y-axis. Starting at zero, zero, each data point is represented at the corner of the rectangle formed by these two lengths. By convention, the first coordinate is measured on the x-axis and the second on the y-axis.

When the two variables have such different sizes, the scale on one axis has to be adjusted to fit the paper. Let's re-scale the x-axis. The points plotted here correspond to the values in this table. The top line is the x values. The second line is the y values.

Time is a continuous variable. So we can draw a straight line through these points to show all the intermediate points. The equation of the line is y equals 60 times x. This equation is the algebraic representation of the graph.

We can read intermediate values from the line graph. How many minutes is 2.5 hours? Start at naught, go along the x-axis to find 2.5, then up till you hit the line. Use a ruler or your finger to find the same height on the y-axis, and read off 150. 2 and 1/2 hours is 150 minutes.

We know what x represents in this graph. It is time in hours, measured on the x-axis. And y is the time in minutes. The number 60 is the gradient. It's defined as the ratio, increase in y divided by increase in x.

The gradient is important for describing covariance. It tells you exactly how y changes when x increases by 1. Here, the gradient is a constant. It's always 60.

On a graph, the gradient also tells you the slope of the graph. A constant gradient means that the graph of the function is a straight line. This graph shows a proportional relationship, of the kind you met in an earlier week. The graph of a proportional relationship is always a straight line that passes through zero, zero.

To summarise, then. You've looked at different ways of representing covariance. You might find a real life object that measures both variables. Or you can visualise specific pairs of equivalent numbers. You can put these pairs of numbers in a table as coordinates. You can visualise lengths growing together or on the two axes of a graph. You can write a rule in words and symbols for a function machine, or as an algebraic equation for the graph.

Learners should meet all these representations. One way of distinguishing types of covariance is to think about the gradient of the graph. The number that tells you how much y changes when x increases by 1.

Learners in the middle years should learn to recognise examples and the graphs of linear and proportional relationships and examples of non-linear relationships. They'll work with equations for straight line graphs only.

End transcript: Video 2 Different representations of covariance

Show transcript|Hide transcript

Video _unit6.3.1 Video 2 Different representations of covariance

Interactive feature not available in single page view (see it in standard view).

Discussion

Time in hours is usually considered as a discrete variable since we only write whole numbers of hours (except perhaps half hours)

Time in minutes could be either discrete or continuous.

An age is discrete as you are likely to measure it in years or in years and months, but not with an accuracy of days or hours.

Number of people and chocolates are discrete variables.

You might have thought of graphs, or lines growing together, or imagined groups of chocolates.

3.2 Increasing and decreasing graphs

The Open University — Wed, 01 May 2019 17:18:55 GMT

In the Video 2, the graph of time in minutes against time in hours was described as increasing, since whenever the -variable increases, the y-variable does too.

It is also possible for graphs to be decreasing. The definition for a decreasing graph is: whenever the -variable increases, the y-variable decreases.

Most graphs are neither increasing or decreasing.

Linear graphs

The Open University — Wed, 01 May 2019 17:18:55 GMT

In the video you saw an example of a linear (straight-line) graph (Figure 11).

If a linear graph is increasing then its gradient is positive, since when increases by 1, y changes by a positive number. If a linear graph is decreasing, the gradient is a negative number.

What happens when the gradient of a linear graph is 0? The y-value does not change when increases and so the graph is horizontal.

Figure _unit6.3.1 Figure 11 Gradients of linear graphs

Activity _unit6.3.2 Activity 8 Function detective

Allow 5 minutes

Recall the variables from Activity 7:

age / number of hours of sleep
number of people / how many chocolates each person gets
sharing a box of 24 chocolates
your age / your elder sibling’s age.

Match each pair with one of the graphs in Figure 12 below.

Is each graph increasing, decreasing or neither?

Figure _unit6.3.2 Figure 12 Four graphs

Discussion

Graph A

You should have found that graph A showed the relationship between the number of people sharing a box of 24 chocolates and how many chocolates each person gets. One way of checking this is to look for the points (1,24), (2, 12), (3, 8) that all lie on the graph. The graph is decreasing. Notice also that this graph is shown as a set of discrete points since the number of people and the number of chocolates are both discrete variables.

You could have chosen either variable for the x-axis (and the other for the y-axis) because this graph is the graph of a reciprocal function. The rule is that:

the number of people sharing a box of 24 chocolates x how many chocolates each person gets = 24

In symbols we could write that xy = 24

Or y = 24/x

Any function where y is a constant divided by x is a reciprocal function. A reciprocal function does not have a constant gradient since the slope changes along the graph.

Graph B

Graph B shows your age (on the x-axis) and (on the y-axis) the age of an (imaginary) sibling who was 3 years old when you are born. So it starts at (0,3). This point is called the y-intercept: it is the point where x takes the value zero. The graph is increasing. The gradient of the graph is 1 since your sibling’s age increases by 1 for every year you gain. The graph shows a linear function since it is a straight line with a constant gradient, but it is not a proportional relationship (see Week 3) because it does not start at (0,0). It is drawn as a line graph since age can be considered as a continuous variable.

Graph C

This graph is neither increasing or decreasing since there are intervals (between ages 20 and 60) in which the x-value increases but the y-value stays the same. It represents age on the x-axis and recommended hours of sleep on the y-axis. It only shows some data points, not joined by a line, so the variables are being treated as discrete variables. However, in the context we are clearly meant to use the data at the given ages to infer (make predictions about) the sleep needs at other ages.

It is unlikely that there is a simple mathematical function that will describe this graph.

Graph D

This graph is not one of those shown. It is an example of a piece-wise linear function, since the different sections are each straight lines. It shows weekly salary on the x-axis and how much income tax you pay on the y-axis (using the 2018-19 tax bands). The curriculum in England suggests that learners should be able to read information from graphs in financial contexts. Research suggests that it is necessary to discuss the context with learners before asking them to interpret graphs. Most 11-14 year olds would not be familiar with the idea that employers take tax out of your weekly pay.

So far we have met

the linear graph which increases regularly so we can calculate a gradient. It has equation y = mx + c where m is the gradient and c is the (starting) value of y when x is zero. These graphs typically occur for costs, for travel or for conversion graphs are linear.
the piece-wise linear graph which has straight line sections. It does not have a simple equation. These graphs occur when there are different types of cost or travel combined on one graph.
the reciprocal graph with equation y = k/x .This graph typically occurs when sharing a constant amount k.

There is one final graph that we will meet in the next section.

3.3 The exponential function

The Open University — Wed, 01 May 2019 17:18:55 GMT

In the next two activities you will meet one last important type of function and graph: the exponential function.

Activity _unit6.3.3 Activity 9 Word of mouth

Allow 10 minutes

Figure _unit6.3.3 Figure 13 Word of mouth

Three friends share a secret. On day 2 they each tell one new person.

Then every day for a week, everyone who knows the secret tells it to one new person.

How many people know on day 2?

How many people know on day 3?

Estimate how many people will know the ‘secret’ on day 8.

Discussion

On day 2, 6 people know the secret.

On day 3, 12 people know.

This is a sequence you met before: 3, 6, 12, 24, 48. The term-to-term rule is ‘double each term to get the next’. The position-to-term rule is 3 × 2^n-1.

By day 8, a large number of people will know. You can use the formula or a graph to find the value (Figure 14).

Figure _unit6.3.4 Figure 14 Sharing secrets

In this problem, the number of new people who learn the secret each day is equal to the number who know it already. This makes it an example of an exponential function. The characteristic feature of these functions is that their rate of change is proportional to their value. In more everyday words, this means that the higher the y-value, the more it grows.

It is worth comparing this with a linear function, which grows at a constant rate.

This feature lies behind the English phrase ‘growing exponentially’ which is used to emphasise rapid growth. It also makes exponential functions difficult to graph. The scale on the y-axis has to be large to include the points for higher values of x, but then the y-values for low values of x are not easy to read.

As well as exponential growth you can have exponential decay, which gives a decreasing exponential function.

Activity _unit6.3.4 Activity 10 Exploring exponentials

Allow 20 minutes

Figure _unit6.3.5 Figure 15 Exponential functions

Exponential functions are used in many real-life situations.

In biology, they describe the spread of diseases and the growth of populations.
In engineering, they describe waves, and how movement dies away.
In finance, they describe how money grows when it is invested at a constant rate of interest.

Although exponential functions appear on the school curriculum for the middle years, learners are expected to plot and read their graphs and not to work with their equations. This has been included only for your own knowledge asa teacher.

The role of teachers is to support learners in gaining a good mathematical understanding of graphical representation in a range of contexts. Learners need to be able to choose scales and plot x-values and y-values on a graph. They need to find the y-value for a given x-value and vice versa. They also need to interpret change on a graph. They need to know whether the graph is increasing or decreasing, and whether changes are constant or can vary. The linear, piece-wise linear, reciprocal and exponential graphs are all functions that illustrate different kinds of change and this is their value for teachers.

4 This week’s quiz

The Open University — Wed, 01 May 2019 17:18:55 GMT

Check what you’ve learned this week by doing the end-of-week quiz.

Week 5 practice quiz

Open the quiz in a new tab or window and come back here when you’ve finished

5 Summary of Week 5

The Open University — Wed, 01 May 2019 17:18:55 GMT

In Week 5 you have looked at functional thinking and how this is introduced using the idea of the function machine which works by operating on an input to produce an output. This led to a consideration of number sequences where the input is the position in the sequence and the output is the number (or term) at that position. Finally the idea of covariance was introduced where there are two quantities which vary together according to a relationship. The two quantities can be plotted on a graph and the shape of the graph is determined by the function itself. Although the concept of the function is a sophisticated one, the approaches you met in week five show that it is possible to carefully introduce it to learners in the middle years.

You should now be able to:

introduce learners to the concept of functions using concrete and pictorial representations (such as the function machine) and to develop this towards more abstract representations of functions
use other representations, with which the learners are familiar, to build on their understanding of functions
explain to learners how two unknowns can vary together according to the rule (or function) connecting them.

Next week you will learn about the teaching and learning of geometry, including how shapes are defined according to their properties and problem solving in geometry.

You can now go to Week 6.

1 How children think and reason about geometric shapes

The Open University — Wed, 01 May 2019 17:18:55 GMT

The study of geometry has its foundation in the learner’s experience of the environment in which they live. More than any other area of mathematics we learn about shape and space in an intuitive way through our experience. By the time children go to school, they have already learned much about shapes and have developed spatial perception in a natural way by interacting with their environment.

It would seem appropriate that any approach to learning about the geometry of shape must build on the intuitive understanding which is gleaned from our lived experience. Nevertheless, even though children come to school with an impressive understanding of shape and space pertaining to their environment, most encounter difficulties when learning about geometry.

1.1 Progression in geometric thinking

The Open University — Wed, 01 May 2019 17:18:55 GMT

A Dutch educator called Pierre van Hiele showed how children’s understanding of shape develops through increasing levels of sophistication.

At first, children learn to recognise shapes in a holistic way and to name them. This usually happens in early primary school. As they progress, children learn to describe shapes by their properties and to recognise a shape from a list of properties. These children tend to be older primary to younger secondary age, and some may never progress beyond this level.

In the third level, children or learners are able to reason more abstractly, to work out how one property of the shape generates other properties, and to construct a concise definition for a shape.

Later levels describe much more advanced geometrical reasoning which is appropriate for higher school and university study.

Thinking holistically

The Open University — Wed, 01 May 2019 17:18:55 GMT

In their early school years, learners have typically been presented with shapes and they have learned their names (Figure 1). However, they will also have learned some implicit things about these shapes. One of these is orientation or ‘right way up-ness’.

Figure _unit7.1.1 Figure 1 The common presentation of four basic shapes

It is common for children in early years schooling to be presented with shapes which are upright, in the case of squares and rectangles, and for triangles to sit on a horizontal base. So, quite often, learners decide that the orientation of the shapes is an important property and do not accept shapes in orientations which are unusual. As an example of this, consider the two squares in Figure 2 below, which are presented in different orientations.

Figure _unit7.1.2 Figure 2 A square in two different orientations

It is common for the first of these two shapes to be perceived as a square, whereas the second shape is perceived as a ‘diamond’ because it sits on its corner.

One way of dealing with this is to suggest that the learner turns the paper round, so that they can see the shape ‘the right way up’. This is more difficult if the shape cannot be turned round, but the suggestion can be to turn the head so as to look at the shape ‘the right way up’.

Moving on to description

The Open University — Wed, 01 May 2019 17:18:55 GMT

Learners develop an intuitive feel for the look of a shape: the squareness of a square, the circular nature of a circle, and so on. If learners only see and experience ‘typical’ examples of each kind of shape then they may not accept shapes which are in unusual proportions or orientations. For example, the shapes in Figure 3 are all kites but only the first one is typical of the orientation and proportion shown in textbooks and on classroom posters.

Figure _unit7.1.3 Figure 3 Four kites in different proportions and orientations

You can be sure that these shapes are indeed kites if the properties of a kite hold true for these shapes.

Activity _unit7.1.1 Activity 1 Properties of a kite

Allow 5 minutes

Write down as many properties of a kite as you can think of. Remember to use angle sizes, side lengths and symmetry.

Discussion

Here are some properties of the kite:

Four sides, i.e. the kite is a quadrilateral.
Two pairs of adjacent equal length sides (adjacent means the sides are next to each other).
One pair of opposite equal angles.
One line of symmetry.
No rotational symmetry or rotational symmetry of order 1 (i.e. there is only one position where you can fit the kite in a 360⁰ turn).

Once learners are able to describe shapes by listing their properties they will accept unusually orientated or proportioned shapes such as the kites in Figure 3. They can progress to solving simple geometrical problems using the properties of the shapes that they know.

Activity _unit7.1.2 Activity 2 Is this a kite?

Allow 10 minutes

Figure _unit7.1.4 Figure 4 Is this shape a kite?

Discussion

ABCD is a kite.

Adjacent sides AB and AD are equal in length (2 units).

Adjacent sides BC and BD are equal in length (they are both the diagonals on 3 by 1 rectangles).

AC is the line of symmetry. (Points A and C are on the line. Points B and D are both one diagonal of a square away from line AC.)

Activity _unit7.1.3 Activity 3 Beginning to reason abstractly

Allow 10 minutes

In Activity 1 you listed the properties of a kite. Now work out a definition of the kite which uses only two or three of its properties. These properties are known as the necessary and sufficient conditions. There is more than one way to write a definition of the kite.

Discussion

Here are some definitions of the kite using necessary and sufficient conditions.

A quadrilateral with two pairs of adjacent equal sides.
A quadrilateral with a line of symmetry and a pair of opposite equal angles.

Did you think of any others?

1.2 Defining shapes using flexible definitions

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will consider how more flexible definitions of shapes can be used to include other kinds of shapes.

Being able to write a definition of a shape is an example of analytical thinking. As learners’ reasoning develops, they are able to define shapes in different ways and to develop definitions which are more inclusive. As an example of this, consider the rectangle and the square (Figure 5).

Figure _unit7.1.5 Figure 5 A rectangle and a square

Skip transcript: A square being ‘pulled’ into a rectangle

What happens if you were to take one side of the square and pull it? The following video shows this happening.

Download this video clip.Video player: A square being ‘pulled’ into a rectangle >

Video _unit7.1.1 A square being ‘pulled’ into a rectangle

Transcript: A square being ‘pulled’ into a rectangle

INSTRUCTOR:

Here is a square. If I pull on one of its sides, I can make this square into a rectangle. I can make the square into different rectangles whose width is the same as the side of the square, but whose length changes.

End transcript: A square being ‘pulled’ into a rectangle

Show transcript|Hide transcript

Video _unit7.1.1 A square being ‘pulled’ into a rectangle

Interactive feature not available in single page view (see it in standard view).

You have just watched a square being pulled into a rectangle. Might a square be thought of as a special kind of rectangle? (In mathematics we call this ‘a special case’.)

Activity _unit7.1.4 Activity 4 Writing definitions

Allow 5 minutes

Write down a definition of the rectangle.

Write down a definition of the square.

Activity _unit7.1.5 Activity 5 More definitions

Allow 10 minutes

Below are two definitions of a rectangle, with a subtle difference between them, and one definition of a square.

Is it possible to include the definition of a square into either of the two definitions of a rectangle?

Definition 1 of a rectangle

A rectangle has four sides and four right angles. It has two longer sides of the same length, which are opposite each other, and two shorter sides of the same length which are opposite each other.

Definition 2 of a rectangle

A rectangle has four sides and four right angles. It has two pairs of equal sides which are opposite each other.

Definition of a square

A square has four sides which are the same length and four right angles.

Discussion

The definition of a square cannot be included in definition 1 because its definition of a rectangle stipulates that there are two sizes of equal sides: the longer sides and the shorter sides.

The definition of a square can be included in definition 2 because its definition of a rectangle says that there are two pairs of equal and opposite sides. This also includes the case when these two pairs of equal sides are also equal to each other, in which case the rectangle is also a square.

The understanding that some classes of shapes can be included in other classes of shapes is called the concept of inclusion. So, for example, squares can be seen to be special rectangles. However, in order to accept the concept that squares are special rectangles, a more inclusive definition of the rectangle, such as definition 2, needs to be used.

Activity _unit7.1.6 Activity 6 Including some shapes as special cases of other shapes

Allow 5 minutes

Try to identify other inclusive relationships. For example, is there a definition of the kite which would include the rhombus? Can rectangles be thought of as special cases of the parallelogram?

1.3 The visual or physical representation of shapes

The Open University — Wed, 01 May 2019 17:18:55 GMT

If you consider a shape, such as a square or a circle, you have a mind’s-eye picture of the shape which includes the ‘squareness’, or the ‘circleness’, and this shape may be filled in (with colour) in your mind, and it may have a particular orientation. You may draw a diagram of this shape with a pencil and the outside edges are likely to have some width (or you could not see them). If you have a square or circular tile then the tile will have depth (or you could not pick it up).

The diagram and the tile are representations of the shape and as such are examples of what the shape can look like. In contrast, the mathematical shape itself is entirely abstract and is described using its geometric properties. Orientation and thick pencil lines are not included in those properties. However, teachers and learners need to work with representations of shapes as if they are mathematical objects because it is all they have to work with. It is worth being aware of the differences because learners’ attention may be distracted by particular features of the physical representations of the shapes.

2 Investigating the properties of triangles and quadrilaterals

The Open University — Wed, 01 May 2019 17:18:55 GMT

In the previous section you read about the properties of some shapes and how combinations of those properties form the definitions of those shapes. Triangles and quadrilaterals are important because they form the basis of other shapes and are generally included in the school curriculum for the middle years.

2.1 Triangles

The Open University — Wed, 01 May 2019 17:18:55 GMT

Triangles are the most basic of the polygons (a polygon is a straight-sided shape). They have three sides, the smallest number of sides it is possible to have. Triangles also make a rigid structure, as any engineer will know.

You will know the names of the triangles which have further properties in addition to having three sides (Figure 6).

Figure _unit7.2.1 Figure 6 Types of triangle

In addition, the isosceles right-angled triangle has a right angle, two equal sides, and two equal angles of 45⁰ each.

Acute-angled triangles have three angles all smaller than a right angle.

Obtuse-angled triangles have one angle that is bigger than a right angle.

2.2 Quadrilaterals

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will explore how certain quadrilaterals can be made from triangles.

Activity _unit7.2.1 Activity 7 Making shapes out of two congruent right-angled triangles

Allow 10 minutes

Draw and cut out two congruent right-angled triangles. (Congruent means they are exactly the same size and shape, same angles, etc.)

Can you make the following shapes by placing the congruent right-angled triangles together along two of their edges (i.e. no overlapping)?

2 different isosceles triangles
2 different parallelograms
1 rectangle
1 pentagon
1 hexagon

Discussion

Figure _unit7.2.2 Figure 7 Shapes made placing two right-angled triangles together

Activity _unit7.2.2 Activity 8 Reflecting a triangle in one of its sides

Allow 10 minutes

Draw a triangle (not a special triangle) and then draw a reflection of that triangle in one of its sides. Keep both the original triangle and the reflected triangle which should now make a quadrilateral. What kind of quadrilateral do you get?

Then do the next activity and read the discussion afterwards.

Activity _unit7.2.3 Activity 9 Reflected triangle in Geogebra

Allow 10 minutes

Click on this link https://www.geogebra.org/ m/ U6azsQb6 which takes you to a Geogebra file that contains one triangle which has been reflected in one of its sides. ‘Drag’ the resulting quadrilateral to investigate which shapes you can make. (If you have difficulty in opening the link, go to www.geogebra.org and search for ‘triangle reflected in one of its sides’.)

Discussion

You can drag the figure at the link above if you have clicked on the arrow tool (which is the default tool when you open a Geogebra file) and then you drag the figure on the screen. Were you able to make any of the following?

a kite
a rhombus
a square
a isosceles
an triangle

Activity _unit7.2.4 Activity 10 Splitting quadrilaterals

Allow 5 minutes

Which quadrilaterals can be split into two congruent triangles? Complete the table in Figure 8. The third column refers to whether the triangles are reflected or rotated in the quadrilateral.

Figure _unit7.2.3 Figure 8 Splitting quadrilaterals into two congruent triangles

Discussion

The table can be completed as shown in Figure 9.

Figure _unit7.2.4 Figure 9 Solutions to splitting quadrilaterals

These tasks are a novel way to approach learning about triangles and quadrilaterals. They encourage learners to think about the shapes and their properties. Many textbook exercises consist of questions asking learners to recall shape properties, without requiring learners to think deeply about them.

The special quadrilaterals – those we have names for – are special because they have interesting properties such as symmetry, which relates to the triangles which make them up.

3 Developing problem-solving skills in geometry

The Open University — Wed, 01 May 2019 17:18:55 GMT

So far this week, you have read about children’s geometrical thinking as it develops through the early to the middle school years. This has been linked to the description of the properties of shapes. You have looked at definitions of shapes and how these can be written in an inclusive way to include some shapes as special cases of others. You have considered triangles and how these can generate special quadrilaterals by reflecting a triangle in one of its sides or rotating the triangle using a half-turn around the midpoint of one of its sides.

In this section you will think about developing learners’ problem-solving skills in geometry. Many problems in geometry are entirely abstract from the point of view that there may be no practical real-world application. However, geometry can be approached in a practical way and this makes it accessible and engaging for learners. Geometry does also have real-world applications in nature, in design and architecture for example.

Three problems will be posed that you will be asked to have a go at answering. After each one you will consider how to best pose these problems for learners, including the resources needed and how to manage these in the classroom context.

3.1 Polyominoes

The Open University — Wed, 01 May 2019 17:18:55 GMT

The activities in this section can also be used to consider reflection and rotation symmetry.

You have probably seen dominoes, which are made of two squares joined together along their complete edges (Figure 10). There is only one domino because turning it round into a different orientation does not count as making another domino. A domino has two lines of symmetry (horizontal and vertical) and rotational symmetry of order 2 (2 positions where the domino can fit within a 360⁰ turn).

Figure _unit7.3.1 Figure 10 A domino

As you might imagine, triominoes are made of three squares placed along their complete edges. ‘Tri’ at the beginning of a word denotes three (like ‘triangle’ for a three-sided shape and ‘tricycle’ for a three-wheeler cycle).

There are two different triominoes (Figure 11). Remember that turning a triomino round or flipping it over does not create a different triomino.

Figure _unit7.3.2 Figure 11 The two triominoes

Activity _unit7.3.1 Activity 11 The symmetry of triominoes

Allow 5 minutes

Describe the symmetry of each of the two triominoes.

Does each triomino have reflection symmetry and, if so, where is the line of reflection?
Does each triomino have rotation symmetry?

Discussion

The I-shape triomino has two lines of symmetry and rotation symmetry of order 2.

The L-shape triomino has one line of symmetry and rotation symmetry of order 1 (i.e. no rotation symmetry).

Activity _unit7.3.2 Activity 12 The tetrominoes

Allow 5 minutes

Tetrominoes are made by joining four squares together along their complete edges. Try to find all of the possible tetrominoes.

How can you be sure that you have found them all?

Describe their symmetries.

Discussion

Figure _unit7.3.3 Figure 12 The tetrominoes

Activity _unit7.3.3 Activity 13 The pentominoes

Allow 10 minutes

Now try to find all 12 pentominoes (5 squares joined along their complete lengths).

Several challenges can be posed using the 12 pentominoes. It is useful to be able to give them labels, especially since many of them look like letters of the alphabet (with a little imagination). For example, one set of labels uses the letters F, I, L, N, P, T, U, V, W, X, Y and Z. See if you can match these letters to the pentominoes you made.

One challenge is to use all 12 pentominoes to make a 10 by 6 rectangle. 5 × 12, 4 × 15 and 3 × 20 rectangles are also possible. Other challenges are to make animals – a penguin, camel, dog and kangaroo are all possible.

Individual pentominoes can be used to make tessellations (or tiling patterns). For example, the P-shape and the F-shape can be tiled.

Pentominoes as an activity with learners

The Open University — Wed, 01 May 2019 17:18:55 GMT

I remember the first time I met this activity. I was working as a Special Needs teacher in the mathematics department of a large college and was supporting the class teacher of a low attaining group of 15-year olds. She had found the activity in a publication and had printed it out. The activity engaged the learners and they were all able to make a start at it. No one individual found all of the twelve pentominoes but between the whole class all twelve were found and displayed on the board.

Activity _unit7.3.4 Activity 14 Preparing a lesson using pentominoes

Allow 20 minutes

Imagine that you are going to use the pentominoes task with the group of 15-year-olds. List the equipment you will need for the lesson.

Discussion

Think about how you will introduce the task to the learners. Can you expect a whiteboard in the room, or an interactive whiteboard? Will you draw on the board? Will you make tiles which can be moved around?

Think about the learners investigating the task for themselves and the equipment they are likely to need. Square paper is useful. It may be that learners are given square tiles to use to create the pentominoes which can then be drawn on the square paper.

Now read this:

In the lesson with the 15 year old low attaining group the class teacher began by drawing a domino and talking about it as two squares put together along their edges. She established with the class that it had to be whole lengths of the edges placed together so that

Figure _unit7.3.4 Figure 13 A lesson about polyominoes

She told the class that triominoes were made from three squares placed together along their edges, gave them some thinking time and then got learners at the board to draw two possible different triominoes.

The learners were next asked to find as many tetrominoes as they could and to draw them on squared paper. Some time was given for this and then the teacher asked for volunteers to draw the results on the board. She drew attention to the systematic way of finding all of the five tetrominoes and how this helps us to be sure that we have found all possible. It also gave her the opportunity to talk about congruency, which is where we have two tetrominoes of the same shape, if only we could cut them out and sit one on top of the other. A tetromino which is congruent to one of the others is not allowed as they all need to be different.

In the last part of the lesson the learners were set to task finding the twelve pentominoes, which they drew on squared paper.

The lesson was engaging for the pupils because it was simple enough for them all to make a start. They could find more of the polyominoes by drawing, using visual intuition. the task encouraged them to be logical and systematic as a way of identifying all possible shapes. The class experienced individual working but also collaborative working when they came together frequently as a group to discuss whether they had found all shapes.

3.2 Ideas for tasks using four cubes

The Open University — Wed, 01 May 2019 17:18:55 GMT

So far, only 2D shapes have been considered but now you will think about 3D shapes or models.

Most mathematics departments have sets of interlocking cubes, or multi-link cubes as they are sometimes known (Figure 14). The bigger ones which are 2 cm along their edges are best to use as they are easy to handle.

Figure _unit7.3.5 Figure 14 Four multi-link cubes

There are several tasks which you can ask learners to do with four interlocking cubes.

Make a four-cube model and hide it behind a book or other appropriate hiding place. Now try to describe it to another person so that they could make the same model using four cubes.
Find all of the different models which can be made using four interlocking cubes. How can you be certain that you have them all?
Which four-cube model has the largest or smallest surface area?
Draw the models on isometric paper.
Draw the plan and elevations of each model.
Choose one of the models, devise its net and make it up using card.

Activity _unit7.3.5 Activity 15 A task using four cubes

Allow 25 minutes

Choose one or more of the tasks above to try for yourself (having first procured four cubes). Then read the following suggestions and points regarding each suggested activity.

Make a four-cube model and describe it to another person so that they could make the same model using four cubes

Learners may find it helpful to use terms such as:

base
next to
above
in a row

Find all of the different models which can be made using four interlocking cubes

As with the pentomino activity, it helps to be systematic. In addition, shapes which are congruent to other shapes are not considered to be different. Think about cutting a 3D-shaped hole to fit the four-cube model. If another four-cube model can be fitted into the same hole then it is congruent and not different.

Figure _unit7.3.6 Figure 15 Three four-cube models

Which model has the largest or smallest surface area?

All of these four-cube models have the same volume because they have the same number of cubes. If the edge is considered to be one unit then the area of the face of a cube is one square unit and the volume of one cube is one cube unit. So, the four-cubed models each have a volume of four-cube units. However, their surface areas, which can be counted and recorded in square units, will be different.

Drawing the models on isometric paper

There is an art to drawing representations of 3D images on isometric paper. Learners may need some advice. Isometric paper is based on equilateral triangles and the orientation of the paper is important. Make sure that the paper looks like the example in Figure 16, with vertical lines of dots. If not, turn it 90⁰.

The first step is to draw one cube:

Draw a rhombus shape, which will represent a square face in perspective.
Next put three parallel lines from each of the three corners to provide the depth of the cube as shown.
Finally complete the cube as shown.

Figure _unit7.3.7 Figure 16 Drawing a cube on isometric paper

Once the learner has mastered the skill of drawing a cube on isometric paper they will be able to draw the four-cube models by extending the original cube to represent cubes added on.

Drawing the plan and elevations of each model

This work is related to the work of architects on plans and elevations for buildings. The plan is the footprint of the model, which sits on the table. The elevations are the front and the side views imagined as if they were flat.

The side and elevations of the (four-cube model) are shown in Figures 17 and 18. Squared paper is useful for this activity.

Extensions to this work could be to provide the plan and elevations for a cube model. Learners then have to make the model and sketch it in 3D on isometric paper.

Figure _unit7.3.8 Figure 17 The four-cube model used to draw the plan and elevations

Figure _unit7.3.9 Figure 18 Plan and elevations for the four-cube model

Devise a net for a four-cube model and make it up using card

This task can be easy or hard, depending on the choice of four-cube models. It is not as difficult as it might first seem if the model is placed on to paper and the learner considers the shape of the base, what needs to be folded up to make the sides and then what needs to be attached to the sides to fold over to make the top. Trial and improvement is a useful strategy since if the net does not work it can be modified and tried again until it does work.

3.3 Enlarging shapes using repeating tiles

The Open University — Wed, 01 May 2019 17:18:55 GMT

The activities in this section can be linked to enlargement as a transformation.

Some shapes can be enlarged when several smaller versions of themselves are laid together (Figure 19). Squares, rectangles and parallelograms all come into this category. Note how many of these shapes fit together to make an enlargement which is two or three or four times as big.

Figure _unit7.3.10 Figure 19 Repeating tiles

Triangles are also repeating tiles. Try this with different versions of a triangle. You can either make a triangle tile from card or using ICT. You may have to rotate some of the triangle tiles.

Activity _unit7.3.6 Activity 16 Using repeat tiles to make a version which is twice the size of the original

Allow 10 minutes

Explore other shapes which can be used as repeating tiles. For example, show how the shape below can be generated from four smaller versions of itself.

Figure _unit7.3.11 Figure 20 An L-shaped tile which can be made from four smaller L-shaped tiles

Discussion

Figure _unit7.3.12 Figure 21 The solution to Activity 16

Repeating tiles as an activity with learners

This task gives the opportunity for learners to create and manipulate cardboard tiles in order to explore which shapes are repeat tiles. Alternatively, the tiles could be produced using ICT. For example, some shapes are generated by the ‘Insert Shapes’ facility of Microsoft Word.

Repeat tiles provide an approach to enlarging shapes because the final result of placing together several repeat tiles must be a correct larger version of the original repeat tile. When four repeat tiles are joined together then the length of each side of the enlargement is 2 times the length of the side of the original tile. In order to create a shape where the lengths of each side are 3 times the original tile, you need to use 9 tiles.

Learners can investigate the number of tiles needed to generate enlargements of 2, 3, 4 and 5 times the original tile.

This can lead to the observation that the factor of enlargement for area is the square of the factor of enlargement for lengths.

4 This week’s quiz

The Open University — Wed, 01 May 2019 17:18:55 GMT

Check what you’ve learned this week by doing the end-of-week quiz.

Week 6 practice quiz

Open the quiz in a new tab or window and come back here when you’ve finished

5 Summary of Week 6

The Open University — Wed, 01 May 2019 17:18:55 GMT

In Week 6 you read how the study of geometry can be very intuitive, drawing on learners’ experience of the lived in world and as such it is a very practical area of mathematics. On the other hand teachers and their learners work with diagrams and physical objects which are themselves representations of abstract geometrical objects. There is a tension between the specific physical representation which can be seen and touched and the abstract concept in geometry (e.g. the triangle on paper represents a whole set of all possible triangles in the abstract).

You should now be able to:

understand that geometrical shapes have properties and that their definitions draw on these properties
use varied activities to support the exploration of geometric shapes, both 2D and 3D
think about how to set up the classroom to facilitate problem solving activities in geometry, including use of practical resources.

Next week you will learn about measures, how these were developed through history, and how to address the appropriate use of measures within mathematics lessons.

You can now go to Week 7

1 Measures and measuring

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will consider different types of measuring scales and read about the history behind the development of measures.

1.1 Measuring scales

The Open University — Wed, 01 May 2019 17:18:55 GMT

Quantities of things are measured using measuring tools such as a ruler or a thermometer. These measuring tools may have numerical scales and the quantity is read from the scale.

Measurement scales can be:

Ordinal scales, when things are put in order, e.g. 1^st, 2^nd, 3^rd.
Interval scales, such as temperature, where equal differences on the scale represent equal differences in temperature. However, there may be no true zero point. This means it is not possible to say that a temperature which is 40⁰ Fahrenheit is twice the temperature as 20⁰ Fahrenheit.
Ratio scales, which satisfy the properties of measurement, in that equal intervals of the scale represent equal intervals of what is being measured and there is a non-arbitrary zero. Examples include length, area, volume, angle and age.

Measurements are thought of as being rational numbers (i.e. whole numbers and fractions or decimals) even though they may represent irrational numbers such as the diagonal of a square or the circumference of a circle which involve numbers such as the square root of 2 or π (pi). This links with the idea of approximation in measurement, which will be considered later.

Activity _unit8.1.1 Activity 1 Reflecting on learners’ difficulties with using measures

Allow 5 minutes

Have you observed learners using measuring tools such as a ruler or tape measure, weighing scales, a thermometer or clocks?

What problems or misconceptions were evident?

Discussion

Learners may struggle with understanding that measuring (a length, a weight, a time interval, etc.) means dealing with a continuous quantity which starts with zero on the measuring scale, rather than with 1 as in counting. Research evidence indicates that when students measure lengths, they may be applying a poorly understood procedure, rather than focusing on the correspondence between the units on the ruler (seeing it erroneously as a counting device) and the length being measured (Battista, 2006).

Time measurement is given in analogue form (a round-faced clock with the numbers 1 to 12 on the face where the passage of time is analogous to the turning of the hour and minute hands) and digital form (using numbers to indicate hours and minutes passed). Time has a different base (e.g. time uses base 60 and 24) from decimal (base 10).

1.2 The historic development of measures

The Open University — Wed, 01 May 2019 17:18:55 GMT

The development in the use of measures happened because people wanted to trade (in foodstuffs, amounts of cloth, etc.) and because communities needed to divide areas of land between family groups. Informal measurements began with reference to parts of the body, for example:

The inch was the length from the tip of the thumb to its joint.
The foot was the length of a man’s foot.
The yard was the length from a man’s nose to the tip of the index finger on his outstretched arm.

I know that, if I wanted to buy some cloth from a merchant, I would have sent the tallest man in my family! Hence the need for these measures to be standardised. The first time that measures were standardised in the UK appears to have been in 1215 in the Magna Carta.

Some of the old imperial measures have strange names. For example, 22 yards is equal to one chain. My historian friend tells me that every community had a chain. This was literally a chain made of links, 22 yards long and used to measure the ground when setting out fields.

Imperial measures of length

The Open University — Wed, 01 May 2019 17:18:55 GMT

Below is a list of imperial measures of length and how they relate to each other.

12 inches to 1 foot
3 feet to 1 yard
22 yards to 1 chain (real chains, see previous comment)
10 chains to 1 furlong (still used in horse racing)
8 furlongs to 1 mile
1760 yards to 1 mile.

Imperial measures of weight

The Open University — Wed, 01 May 2019 17:18:55 GMT

Below is a list of imperial measures of weight and how they relate to each other.

16 ounces (oz) to 1 pound (lb)
14 pounds to 1 stone
8 stones to 1 hundredweight (cwt)
20 hundredweight to 1 ton.

Imperial measures of volume (liquid capacity)

The Open University — Wed, 01 May 2019 17:18:55 GMT

Below is a list of imperial measures of volume and how they relate to each other.

20 fluid ounces (fl oz) to 1 pint
8 pints to 1 gallon.

If you are as old as me, you will have had to learn all of these conversions between units of measure! The United Kingdom was ‘decimalised’ in 1971 which meant that the UK changed from using money in bases of 12 and 20 (12 pence in a shilling, 20 shillings in a pound) to the current system of 100 pence in a pound. This change in the measurement units for money happened fairly quickly but metrication (the change from the imperial system to the metric system), which began officially in 1973, remains unfinished. Schools teach the use of metric units but also have to teach learners about imperial measures because there are still areas of public life where these are used. Examples include distance in miles, vehicle speeds in miles per hour (mph) and beer sold in pints.

1.3 The metric system

The Open University — Wed, 01 May 2019 17:18:55 GMT

The metric system is based on grams (g), litres (l) and metres (m).

The prefixes always mean the same. The common prefixes are as follows.

Kilo 1000

1 kg = 1000 g
1 km = 1000 m

Centi 1/100

1 cm = 1/100 metre
100 cm = 1 metre

Milli 1/1000

1 mm = 1/1000 m
1000 mm = 1 m
1 millilitre = 1/1000 litre
1000 ml = 1 litre

Water connects all three measures!

1 cm × 1 cm × 1 cm = 1 cubic centimetre (cm³) and it holds 1 ml of water which weighs 1 gram (g).
10 cm × 10 cm × 10 cm = 1000 cubic centimetres and it holds 1 litre of water weighing 1 kg.

1.4 Useful ‘rough’ metric to imperial equivalences

The Open University — Wed, 01 May 2019 17:18:55 GMT

Below is a list of useful ‘rule of thumb’ equivalences between metric and imperial measures.

10 cm is about 4 inches
30 cm is about 12 inches = 1 foot
8 km is about 5 miles
A litre of water is a pint and three-quarters.
Two and a quarter pounds of jam, weighs about a kilogram.

1.5 Everyday measures

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will use your everyday experience to think about appropriate use of measures.

Activity _unit8.1.2 Activity 2 Having a sense of common objects and their measures

Allow 10 minutes

It helps to have a sense of what a metre looks like, what a kilogram feels like, and so on. Take a look in the kitchen cupboard and list items which are measured in grams, kilograms, millilitres, centilitres, litres and metres.

Discussion

It can help children to consider everyday items and their measures. Here are some common measures (clearly they are approximate).

A packet of crisps weighs 25 grams.
An apple weighs about 100 grams.
A bag of sugar weighs 1 kilogram.
A family car weighs 1 tonne. (Note: imperial tons and metric tonnes are similar in size.)
A teaspoon holds 5 millilitres.
A bottle of wine holds 75 centilitres.
A bottle of fizzy drink holds 2 litres.
A ruler is 30 centimetres long.
The room is 2½ metres high.

Activity _unit8.1.3 Activity 3 ‘The tailor’s rule of thumb’

Allow 15 minutes

Figure _unit8.1.1 Figure 1 Gulliver in Lilliput

‘The tailor’s rule of thumb’ was said to be used to make shirts and trousers in the past. It is quoted in the novel Gulliver’s Travels by Jonathan Swift:

The seamstresses took my measure as I lay on the ground, one standing at my neck, and another at my mid-leg, with a strong cord extended, that each held by the end, while a third measured the length of the cord with a rule of an inch long. Then they measured my right thumb, and desired no more; for by a mathematical computation, that twice round the thumb is once round the wrist, and so on to the neck and waist, and by the help of my old shirt, which I displayed on the ground before them for a pattern, they fitted me exactly.(Swift, 1726, p73)

Work out how the Lilliputian seamstresses calculated Gulliver’s wrist, neck and waist measurements.

Discussion

The mathematical computation is:

‘Twice round the thumb is once round the wrist

Twice round the wrist is once round the neck

Twice round the neck is once round the waist.’

By measuring round the base of the thumb, a tailor can get a rough estimate of wrist, neck and waist measurements.

An interesting classroom activity is to get learners to see how accurate this is with string. However, caution is needed where learners have issues about body image.

1.6 Extending measures to compound measures

The Open University — Wed, 01 May 2019 17:18:55 GMT

Compound measures are comprised of at least two other measures. Perhaps the most commonly used example is speed which is the distance travelled in one unit of time (i.e. it uses a combination of length and time measures). It is measured in miles per hour, km per hour, metres per second, etc.

When introducing compound measures to my learners I used to pose this question:

‘Which is heavier: a ton of lead or a ton of feathers?’

My learners, being smart young people, would think about this question and then say ‘They weigh the same because you said they weighed a ton’. I would then ask:

‘What is the obvious difference between the ton of feathers and the ton of lead?’

The answer is that the ton of feathers takes up much more space than the ton of lead. This leads to a consideration of volume and weight. Lead is denser than feathers as it is more compact. We would use this to work out that density is the weight per unit volume of a substance.

One year I had a thoughtful group of learners who replied that if a ton of lead fell on you it would probably kill you whereas you would be likely to survive having a ton of feathers falling on you. Cue the class clown brilliantly miming someone climbing out of a ton of feathers!

2 Reading measures and accuracy

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will look at different types of measuring scales and consider appropriate degrees of accuracy in measurement.

2.1 Measuring scales

The Open University — Wed, 01 May 2019 17:18:55 GMT

Reading a length from a ruler or tape measure is straightforward because these measuring devices are generally marked in unit lengths and each unit length is subdivided. A metre ruler may be marked in centimetres, each of which are subdivided into tenths or 0.1 of a centimetre.

The tape measure shown in Figure 2 has inches on one side, subdivided into quarters and sixteenths, and centimetres on the other. Note that the 10s figure for centimetres is only marked for every 10 centimetres.

Figure _unit8.2.1 Figure 2 A tape measure showing inches and centimetres

Reading temperature from thermometers is usually straightforward because they are marked in degrees Celsius (⁰C) and/or degrees Fahrenheit (⁰F) (Figure 3). However, although each gradation may be a unit length, they may be numbered in 20s, 10s or 5s.

Figure _unit8.2.2 Figure 3 A thermometer showing degrees Fahrenheit and Celsius

Reading a weight from a weighing scale may be more challenging. Some scales may be marked in kg or lb but the gradations might not be in the subunits. It may be necessary to count how many intervals a unit has been divided into and enact a division to find out what each subdivision represents. See the picture of the scales in Figure 4.

Figure _unit8.2.3 Figure 4 An old-fashioned weighing machine

Reading the amount of fuel left in the tank of a car is a case in point. Fuel gauges rarely tell you how much fuel is in the tank. Instead, they tell you what fraction of a full tank is left (Figure 5).

Figure _unit8.2.4 Figure 5 A fuel gauge in a car

It is worth saying that many measuring tools are digital and provide a measurement in digits rather than on a scale. Time measurement provides a common example. We continue to measure time using analogue clocks, where the passage of time is reckoned by the position on the clock face of the hour-hand and the minute-hand (Figure 6). Yet we have become used to digital clocks which provide the time as a series of digits (Figure 7).

Figure _unit8.2.5 Figure 6 An analogue clock

Figure _unit8.2.6 Figure 7 A digital clock

2.2 Degrees of accuracy in measurement

The Open University — Wed, 01 May 2019 17:18:55 GMT

Depending on the context, it may not always be necessary to give an exact answer to a measurement. At other times, it is actually more acceptable to have rounded the amount to an appropriate degree of accuracy. The population of countries is often quoted in millions because the exact figure changes constantly. For example, the population of the UK is ‘about 65 million’.

It can also depend on what you are going to do with the measurement. For example, when measuring a window for curtains, the width to the nearest 5 cm would be more than sufficient. But if you are measuring the same window for a replacement pane of glass, a much higher degree of accuracy would be needed.

It should be understood that measurement is continuous, so lengths, weights, etc. can take any value on the number line including decimals and fractions. This is in contrast to amounts which are counted, such as the number of sweets in a jar, which can only take whole number values. However, large numbers of items to be counted may act as continuous measures, such as national population sizes.

Accuracy is often quoted using the number of decimal places. If you need to round a measurement to 1 decimal place that implies to the nearest one-tenth. If you need to round a measurement to 2 decimal places that implies to the nearest one-hundredth.

A number such as 3.8567 lies in between 3.8 and 3.9; numbers with 1 decimal place. You might remember the video on decimal places which explains how the digits after the decimal point denote decreasing powers of ten: tenths, hundredths, thousandths, and so on. Therefore, the only useful digit in deciding whether to round 3.8567 to 3.8 or 3.9 is the second number after the decimal point, i.e. the 5 in this example. You could call this the first unwanted number. Since that number is 5 or more, convention dictates that the number is rounded up to 3.9.

Similarly, with large numbers such as millions, you need to consider the first unwanted number to decide whether to round up or down. So, 65 million people in the UK may have been rounded from, say, 65 345 211. This number lies between 65 million and 66 million but is clearly closer to 65 million. The first digit in the column below one million (i.e. in the 100 000 column) is 3, which is less than 5, and so you round down.

Activity _unit8.2.1 Activity 4 Rounding numbers

Allow 5 minutes

Now have a go at rounding these numbers to the degree specified.

62 to the nearest 10.
14.237 to the nearest 1 decimal place.
14.237 to the nearest 2 decimal places.
465 to the nearest 100.

Discussion

60
14.2
14.24
500

Finally: measurement as a human activity is not an exact science. We are bound by our own skills with measuring and the tools we use for measuring. If we are using a school ruler, for example, we can measure accurately to one decimal place but not more than that. Suppose we do a calculation such as the ones mentioned earlier for the length of the diagonal of a rectangle. We are likely to get a calculator answer with many figures after the decimal point. We cannot measure more accurately than one decimal place and so it makes sense to round the answer to 1 decimal place.

3 Teaching angle measures and angle facts

The Open University — Wed, 01 May 2019 17:18:55 GMT

Learners struggle with the idea of angle being a measurement of turn and also a measure of the gradient between two lines. In particular, learners can be distracted by the lengths of the lines which make up the angles. The angles between the two sets of lines shown in Figure 8 may be considered to be different even though the angle of turn from one line to the other is the same.

Figure _unit8.3.1 Figure 8 Two angles of the same size

3.1 Using a protractor to measure angles

The Open University — Wed, 01 May 2019 17:18:55 GMT

The half-circle protractor is commonly used to measure angles (Figure 9). The inner number scale rises from 0 to 180, from right to left, and the outer scale goes from 0 to 180, left to right. Learners commonly make mistakes because they are not using the correct scale when measuring the angle between two lines. The point to remember when measuring angles is that you need to start at zero on one of the lines and read round on that scale to see the number on the second line.

Figure _unit8.3.2 Figure 9 Using a protractor to measure an angle

In order to read the number from the protractor, it may help to use a ruler to make the lines longer so that they meet the scales on the protractor. Make sure that one of the lines in the angle sits on the zero line. Then, using the scale which starts at zero (the inner scale in this case), read off where the second line meets the scale (24⁰ for this particular angle). If a learner gets 156⁰ for the angle, it is obvious that they have made a mistake. Why is this obvious?

The angle is smaller than a right angle (acute), so the number of degrees must be less than 90.

Degrees in a circle

As you can see from the half-circle protractor displaying 180⁰, it is commonly accepted that there are 360⁰in a circle. No one is entirely sure where this originated, possibly with the Babylonians [2000-1600 BC]. It is certainly worth having at least one plausible explanation ready when teaching this in order to answer the question ‘Why?’

3.2 Angle sum of triangles

The Open University — Wed, 01 May 2019 17:18:55 GMT

The angles of any triangle sum to 180⁰. This can be proved in several ways and can easily be shown to convince learners as a classroom activity.

Activity _unit8.3.1 Activity 5 Demonstrating the angle sum of a triangle

Allow 15 minutes

You will need paper, pen, ruler and scissors.

Draw any triangle and mark the angles at the corners.

Cut out the triangle.

Tear off the corners then put the corner angles together. They should all form a straight line.

If we accept that there are 360⁰ in a circle and 180⁰ on a straight line then we have shown that the three corners add up to 180⁰.

Discussion

This activity can be done in the classroom. If 30 learners can do this activity using 30 different triangles then they will have demonstrated the angle sum of a triangle in 30 different ways. This is not a proof, merely a demonstration that it works.

It is much more fun for learners if they discover angle facts for themselves, rather than being simply told them.

Think of other ways for learners to discover the angle facts using drawing and cutting, and measuring using a protractor.

Figure _unit8.3.3 Figure 10 Simple angle facts

Activity _unit8.3.2 Activity 6 A problem using angle facts

Allow 15 minutes

Below is a design using 8 congruent (i.e. exactly the same size and shape) rhombuses. Calculate the two different angles in the rhombuses.

Figure _unit8.3.4 Figure 11 Design created using eight rhombuses

Discussion

Think about the number of rhombuses which meet in the middle of the design. Their sum must be 360⁰.

Think about the rhombus as a quadrilateral. Its angles must sum to 360⁰. A rhombus has two pairs of equal angles.

Use compasses and a protractor to recreate this design. Begin by drawing a circle and marking off 8 equally spaced points on the circumference. Use these points to draw the bottom halves of the 8 rhombuses.

To find the top points for each rhombus use the compasses opened to the same radius as when you drew the circle. Draw 2 arcs (parts of circles) from each point which will intersect in pairs above the rhombus halves. These intersections are the top points of the rhombuses. Join these to the circumference to complete the rhombuses.

4 Teaching about area and volume

The Open University — Wed, 01 May 2019 17:18:55 GMT

Area is a measurement of the amount of space inside a 2D shape and is reckoned by the number of unit squares inside the shape. Common metric units of area are square centimetres (cm²), square metres (m²) or hectares (ha) for measuring land areas. A hectare is defined as 100 m × 100 m or 10 000 m². Imperial measures of area are square inches, square feet and acres for measuring land areas.

4.1 Area

The Open University — Wed, 01 May 2019 17:18:55 GMT

It is best to introduce the topic of area by considering the inside of shapes as being divided into unit squares with area being recorded as the number of squares and the units of area (e.g. 10 cm²). In contrast, the perimeter of a shape is the total length of its boundary but this is often confused with area by learners.

The next stage in learning about area is to find the area of rectangles which have sides measured as whole numbers. It is straightforward for learners to note that the rectangles contain an array of unit squares (Figure 12). This leads to the general formula for the area of a rectangle = length × width.

Figure _unit8.4.1 Figure 12 A 5 cm by 2 cm rectangle divided into square units

Skip transcript: Video 1 Deriving the area of a parallelogram and the area of a triangle

Using the area formula for a rectangle to derive other area formulas

The Open University — Wed, 01 May 2019 17:18:55 GMT

The area formulas for parallelograms and triangles can be easily derived from the area formula of a rectangle (see Video 1).

Watch this video to see how the area formula of a rectangle can be used to derive the area of a parallelogram and then a triangle.

Download this video clip.Video player: Video 1 Deriving the area of a parallelogram and the area of a triangle >

Video _unit8.4.1 Video 1 Deriving the area of a parallelogram and the area of a triangle

Transcript: Video 1 Deriving the area of a parallelogram and the area of a triangle

INSTRUCTOR:

Step one, draw a rectangle. The area is a times b, which is length times width.

Step two, draw a line from one corner to the opposite side to make a triangle. Step three, cut out the triangle.

Step four, move the triangle to the other side to make a parallelogram. Area equals a times b, which is base times height of the parallelogram.

Step one, draw a parallelogram. Area equals base times height.

Step two, draw one of its diagonals. Step three, cuts along the diagonal.

Check the resulting two triangles are the same by placing one over the other. The area of one triangle is 1/2 the base times the height.

End transcript: Video 1 Deriving the area of a parallelogram and the area of a triangle

Show transcript|Hide transcript

Video _unit8.4.1 Video 1 Deriving the area of a parallelogram and the area of a triangle

Interactive feature not available in single page view (see it in standard view).

Since the parallelogram has been made by moving parts of the original rectangle without losing anything, the area of the parallelogram must be the same as the area of the rectangle.

It is worth bearing in mind that if the area formulas are promoted too quickly, before learners have conceptualised area as being a measure of the inside of a shape, then area measurement becomes a calculation without reference to the geometric situation.

4.2 Converting square metres into square centimetres

The Open University — Wed, 01 May 2019 17:18:55 GMT

If you imagine a tile which is one square metre, then 100 square centimetre tiles would line up along its edge.

You could fit 100 rows of these hundred square centimetre tiles inside the metre square.

That’s 100 × 100 = 10 000 square centimetres in one square metre (Figure 13).

Figure _unit8.4.2 Figure 13 A square metre

4.3 Volume

The Open University — Wed, 01 May 2019 17:18:55 GMT

Volume is a measurement of the amount of space inside a 3D shape and is reckoned by the number of unit cubes inside the shape. Common metric units of volume are cubic centimetres (cm³) or cc, and cubic metres (m³). Examples of imperial measures of volume are cubic inches and cubic feet.

It is best to begin the topic of volume by considering the inside of 3D shapes as being divided into unit cubes with volume being recorded as the number of cubes and the units of volume (e.g. 12 cm³). Volume can be investigated practically using 1 × 1 × 1 cm multilink cubes.

For example, learners can be asked to use multilink cubes to make a cuboid which is 3 cm by 4 cm by 2 cm and to work out how many cubes are needed to make it (Figure 14).

Investigating different sized cuboids should lead to the formula for the volume of a cuboid as

Volume = length × width × height.

As with the area formulas, the volume of other more complicated 3D shapes can be derived from the formula of the volume of a cuboid.

Figure _unit8.4.3 Figure 14 A 4 × 3 × 2 cuboid made of 24 unit cubes

4.4 Converting cubic centimetres into cubic metres

The Open University — Wed, 01 May 2019 17:18:55 GMT

If you imagine a box which is one cubic metre then 100 cubic centimetres would line up along its edge.

You could fit 100 rows of these 100 cubic centimetres in one layer.

There would be 100 layers of cubic centimetres in the cubic metre.

That’s 100 × 100 × 100 = 1 000 000 cubic centimetres in one cubic metre (Figure 15).

Figure _unit8.4.4 Figure 15 A cubic metre

5 This week’s quiz

The Open University — Wed, 01 May 2019 17:18:55 GMT

Check what you’ve learned this week by doing the end-of-week quiz.

Week 7 practice quiz

Open the quiz in a new tab or window and come back here when you’ve finished.

6 Summary of Week 7

The Open University — Wed, 01 May 2019 17:18:55 GMT

In Week 7 you have seen how it is important to be aware of different measuring scales and the various types of measuring instruments. Taking a measurement involves human judgement which is therefore limited to a particular degree of accuracy. For example, we can usually only measure lengths correct to the nearest millimetre.

You have also read a brief summary of the development of measuring systems such as the imperial system and the metric system. Giving learners a history of how measures were developed can be interesting and demonstrate how the use of measures drives from human activity.

You should now be able to:

check that your learners use measuring tools such as rulers, appropriately, for example making sure they put the zero on the ruler at the beginning of the line they are attempting to measure. When measuring an angle, checking that they place the protractor correctly and read from the appropriate scale
help learners understand that area is a measurement (by unit squares) of the amount of space within a 2D shape and that volume is a measurement (by unit cubes) of the amount of space within a 3D shape
derive the area formulas of a parallelogram and a triangle from the area formula of a rectangle.

Next week you will focus on data handling, specifically statistics and probability.

You can now go to Week 8

1 Collecting data and illustrating it in charts

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will consider different methods for collecting and recording data.

In Sections 2 and 3 we will look at analysing and interpreting data.

1.1 Collecting data

The Open University — Wed, 01 May 2019 17:18:55 GMT

Data is a collective name for information recorded for statistical purposes. There are many different types of data, which we will discuss in this section.

Data can be collected in several ways. These include face-to-face or telephone interviews, or asking participants to complete questionnaires in person or by post. Data can also be collected online via an internet search or through other more active data- collecting methods, such as counting the number of passengers on a bus at a specific time of day.

When data is collected from an original source for a specific purpose, this is known as primary data. For example, a learner who is investigating the most popular make and model of cars in their town might count cars which pass a particular set of traffic lights. When data is not collected from an original source, this data is secondary. For example, a learner might find data on goals scored in the football Premier League last year in order to investigate whether strikers from a particular country score more goals on average. Since the data was not collected for this purpose originally and was not collected by the learner themselves, this data is secondary.

1.2 Population and sampling

The Open University — Wed, 01 May 2019 17:18:55 GMT

The population of a survey is everyone who can be questioned in relation to that survey. For example, if a learner wanted to find out the most popular meal in their school canteen, the population of the survey would be all of the students, and possibly also the teachers, in the school.

A sample is a small selection of the population, for example just one class in a school.

There are advantages and disadvantages to using entire populations and samples.

Table _unit9.1.1 Table 1 Advantages and disadvantages of sampling

	Advantages	Disadvantages
Population	Every opinion is included Representative and therefore reliable	Time consuming Expensive
Sample	Quick More cost effective	Only some opinions Selection method could cause bias

Advantages

Disadvantages

Population

Every opinion is included

Representative and therefore reliable

Time consuming

Expensive

Sample

Quick

More cost effective

Only some opinions

Selection method could cause bias

Activity _unit9.1.1 Activity 1 Reflecting

Allow 5 minutes

How could a selection method mean that the results of a sample could cause bias?

Discussion

When data is collected from the entire population, all opinions are taken into consideration. When data is collected from a sample of the population, it is important that the sample is representative of the population.

For example: Joe wants to find out whether motorbikes or cars are more popular, so he has asked 20 people at a motor bike convention which they prefer.

Comment: Joe’s data selection method is clearly flawed as he is much more likely to find that people prefer motorbikes within this sample of the population.Thus they are unlikely to be representative of the wider population.

Considering potential bias when sampling data is part of having data sense.

Sampling

When collecting data from a sample of the population, decisions have to be made about how the sample is chosen. It is important to think about how to ensure the sample is representative of the wider population.

There are many different sampling methods. Here are a few examples.

Convenience sampling is the most simple and straightforward way to collect a sample of data since participants are chosen in the most convenient way and are often asked to volunteer. Statistical investigations carried out by young learners are likely to use this form of sampling, for example when investigating favourite pets earlier in this section, the learners used their own classmates for convenience.

Random sampling can be done by assigning a number to each member of the population and then using a random number generator to select the required number of individuals.

Systematic sampling involves choosing individuals at regular intervals. For instance, if a sample of 20 is needed from a population of 100, every 5^th individual would be used in the sample. This sample will obviously vary depending on how the population is ordered. This is a convenient and simple way to collect a sample but may result in some bias as some smaller groups within the population could be omitted from the sample.

A stratified sample takes into consideration different groups and their proportions within the overall population, using the same proportions in the sample. For example, if a school has more students in year 7 and fewer students in year 11, the sample would take a greater number of participants from year 7 and fewer from year 11. This is done using percentages.

A quota sample does not take into consideration the proportion of each group in the population. Instead, a quota sample may require there to be 20 male and 20 female participants in the sample, regardless of whether there are many more of either gender in the wider population.

Sampling methods are not generally covered in the middle school curriculum. But it is important for learners to be aware that their sampling choices will affect the reliability of their findings when conducting statistical investigations.

1.3 Questionnaires

The Open University — Wed, 01 May 2019 17:18:55 GMT

Questionnaires can be a useful tool when collecting primary data, as they can be quick and cheap to produce. It is important, however, to consider the way questions are written in order to ensure the data collected is representative and unbiased.

Questionnaire questions can require:

yes/no responses
tick boxes with different options
numbered responses
single word responses, or
a sentence to be written.

When writing questionnaire questions, it is important that they:

are unambiguous and easy to understand
do not lead respondents to give particular answers
cover every possible answer
are appropriate for the investigation.

Activity _unit9.1.2 Activity 2 Good or bad questions?

Allow 10 minutes

Look at the questionnaire below.

Can you identify an issue with any of the questions?

If so, rewrite the question to address the issue.

Question 1: What is your favourite colour?

Red
Green
Blue
Yellow

Interactive content appears here. Please visit the website to use it

Question 2: Do you agree that mathematics is the best subject?

Interactive content appears here. Please visit the website to use it

Question 3: How many pieces of fruit do you eat in a week?

0–5
5–10
10–15
15–20
More than 20

Interactive content appears here. Please visit the website to use it

Discussion

Question 1 provided tick boxes for the respondent but not all the possible answers were provided. If the respondent’s favourite colour was purple, they could not respond to this question accurately.

When rewriting this question, you could have included an ‘other’ box. Here you could also have added a space for the respondent to write their favourite colour.

Question 2 is a leading question.

When rewriting this question, you would need to ensure it is unbiased and not leading. You could ask ‘What is your favourite subject?’ and provide options or an empty space for respondents to write in the subject.

Question 3 gives ambiguous options. It is not clear which box to tick when the respondent’s answer is 5 or 10 or 15.

When rewriting this question, change the option boxes so there is no overlap. For example, 0-5, 6-10, 11-15, 16-20, more than 20.

1.4 Tally charts and frequency charts

The Open University — Wed, 01 May 2019 17:18:55 GMT

Tallying is a way of recording data in groups of five.

Recording frequencies in this way means totalling the number of tally marks made (figure 1).

Figure _unit9.1.1 Figure 1 Tally marks

Tally charts can be used to collect and organise primary data.

Look at the tally chart in Figure 2.

Figure _unit9.1.2 Figure 2 A tally chart

The first column contains all of the possible data options. Note that there is an ‘other’ box in case any of the answers do not fit into the first four categories. As with questionnaire questions, it is important that every possible answer can be recorded in a data collection table such as this one.

The second column is for a tally, so that results can be recorded and added to.The final column is the frequency. This is the total for each data category once all the data has been collected.

It is also possible to have a frequency chart without a tally column, like the example shown below.

Table _unit9.1.2 Table 2 What is your favourite ice cream flavour?

Flavour	Frequency
Chocolate	14
Vanilla	5
Strawberry	7
Other	20

Activity _unit9.1.3 Activity 3 Reflecting

Allow 10 minutes

Did you notice that the ‘Other’ category in the example above had the highest frequency?

What conclusions could you draw from this?

Discussion

Sometimes when collecting data, the results can be surprising. Participants sometimes give answers you do not expect.

It may be that there are popular icecream flavours that have not been listed.

In some instances, it is worth introducing further categories once some initial data has been collected.

1.5 Grouping data

The Open University — Wed, 01 May 2019 17:18:55 GMT

When there are a large number of possible outcomes, it may be necessary to group the data.

For example, if a learner is investigating how many DVDs each of their classmates own, the values could vary from 0 to 100 (or more).

Introducing 100+ different possible outcomes into a table is going to be very time consuming and difficult to plot in any kind of graph. Instead, grouping the data in a sensible way can make data collection and analysis much more straightforward.

Below you will see a grouped frequency table which could be used to investigate how many DVDs the learner’s classmates own.

Table _unit9.1.3 Table 3 How many DVDs do you own?

Number of DVDs	Tally	Frequency
0–5
6–10
11–20
21–40
Over 40

Activity _unit9.1.4 Activity 4 Reflecting

Allow 5 minutes

Did you notice that there was no overlap in the class intervals and that all possible outcomes were included?

Did you notice that the class intervals in the example above are not equal?

Discussion

If there is no upper bound for a numerical category, it is common to use ‘over’ or ‘more than’ in order to cover all of the possible responses. Similarly, if there is no lower bound, it is common to use ‘less than’, so the first category could have been ‘less than 5’.

Depending on how the data is going to be used, it might be necessary to use classes with equal widths. We will discuss this later in this week.

1.6 Two-way tables

The Open University — Wed, 01 May 2019 17:18:55 GMT

When data falls into more than one category or a learner is interested in investigating more than one variable, data cannot be organised easily in a frequency table.

Two-way tables are a way of sorting data so that the frequency of each category can be seen quickly and easily.

For example, a learner has asked 20 people about whether they like football and whether they like rugby. The results can be seen in the two-way table below.

From the table, it can be seen that there were 9 people who liked both rugby and football and 4 people who liked neither sport. It can also be seen that there were people who liked rugby but not football, and 5 people who liked football but not rugby.

Table _unit9.1.4 Table 4 Do people who like football also like rugby?

	Like football	Do not like football
Like rugby	9	2
Do not like rugby	5	4

Activity _unit9.1.5 Activity 5 Interpreting a two-way table

Allow 5 minutes

Use the information presented in the two-way table below to answer the following questions.

How many right-handed students are there in the class?
How many students are there in the class in total?

Table _unit9.1.5 Table 5 Do left-handed students like art?

	Left-handed	Right-handed
Likes art	2	12
Do not like art	1	10

Discussion

There are 22 right-handed students in the class as this is the sum of the ‘right-handed’ column.
There are 25 students in total. This is the sum of the ‘left-handed’ and ‘right-handed’ columns.

1.7 Discrete and continuous data

The Open University — Wed, 01 May 2019 17:18:55 GMT

Earlier this week we discussed primary and secondary types of data. Numerical data can be further categorised into discrete or continuous data.

Discrete data is numerical data that can only take certain values. The number of people on a fair ground ride, the score on a pair of dice, or a shoe size are all examples of discrete data.
Continuous data is numerical data that can take any value within a given range. The heights of a group of adults, the lengths of some leaves, or a dog’s weight are examples of continuous data.

Discrete data is counted and continuous data is measured.

Activity _unit9.1.6 Activity 6 Are these measures discrete or continuous?

Allow 5 minutes

A child’s foot length
A child’s shoe size
The time taken to run a race
The number of runners in a race
The number of matches in a box
The speed of a car
Air temperature
Time displayed on a dial watch
Time displayed on a digital watch
Annual salaries of teachers

Discussion

The discrete measures are: examples (2), (4), (5) and (9) since the number of people or matches cannot be anything other than a whole number and the time displayed on a digital watch is restricted to hours, minutes and seconds.

Note that in the case of example (2), a discrete variable need not be restricted to whole numbers (shoes in the UK can be in half sizes).

The continuous measures are examples(1), (3), (6), (7) and (8) since length, time, speed, temperature and time can be measured to any degree of accuracy (there is no limit to the number of decimal places that could be included in the measurement).

Example (10) is rather ambiguous. Strictly speaking, it is a discrete variable, given that there is a basic unit of 1p below which measurements cannot be taken. However, 1p is so small in relation to an annual salary (even the annual salary of a teacher) that in practice it is treated like a continuous variable.

1.8 Grouping continuous data

The Open University — Wed, 01 May 2019 17:18:55 GMT

Earlier you used a grouped frequency table to organise the number of DVDs owned by school students. In a similar way, grouped frequency tables are needed for continuous data.

There is a slight difference in the way the class intervals are organised for continuous data. Since continuous data can take any values within a given range, you need to allow for this in your table, as follows.

Table _unit9.1.6 Table 6 How long do you spend on your mobile phone each week?

Hours (h) spent on phone	Tally	Frequency
0 ≤ h < 10
10 ≤ h < 20
20 ≤ h < 30
More than 30

In this grouped frequency table we are using ‘less than’ and ‘less than or equal to’ notation.

Earlier, when discussing the discrete data example of DVDs, we discussed the importance of having no overlap in the categories. For this reason, the categories were 0–5, 6–10, and so on.

In this example, since time is continuous, we need to allow for values such as 10.5 hours. The first category includes 0 but does not include 10. This means that any value less than 10 hours will be included in the first row. This would include values such as 9.5 or 9.7.

In the second row, 10 is included but 20 is not. This means that any value which is between 10 (including 10) and 20 (not including 20) will go here.

This concept is covered towards the end of lower secondary school but learners in primary school will have seen the notation <, to mean ‘less than’ and ≤ to mean ‘less than or equal’ to. So, we can write 10 ≤ h < 20.

1.9 Stem and leaf diagrams

The Open University — Wed, 01 May 2019 17:18:55 GMT

As well as recording data in frequency tables, results can be recorded and presented in a stem and leaf diagram. This visual representation allows for the data to be grouped in classes and it shows the shape of the data.

Example _unit9.1.1

A group of students challenged their teachers to a speed-texting competition. The 21 participants (12 students and 9 teachers) were each required to send a short text message on a mobile phone. Their times are given in Figure 3 in order of size.

Figure _unit9.1.3 Figure 3 Speed-texting scores

Creating a stem and leaf diagram involves finding the ‘stems’, in this case the tens, and then recording the ‘leaves’ in order.

The stem and leaf diagram for the student times is shown in Figure 4. You can see that 1|9 represents 19 and 4|1 represents 41. There are no values between 0 and 10 which is shown as there are no leaves next to the stem 0. The greatest value in this data set is 41, so there are no values next to the stem 5 either.

Figure _unit9.1.4 Figure 4 An ordered stem and leaf diagram

Activity _unit9.1.7 Activity 7 Creating a stem and leaf diagram

Allow 10 minutes

On paper, have a go at creating an ordered stem and leaf diagram for the teacher’s data from Figure 3.

Discussion

Your diagram should look like Figure 5.

Figure _unit9.1.5 Figure 5 Completed stem and leaf diagram

2 Developing data sense and forming sensible conclusions

The Open University — Wed, 01 May 2019 17:18:55 GMT

So far this week, we have discussed how data can be collected and recorded in frequency tables Later, in Section 3, you will look at how data can be presented in different graphs and charts.

In this section you will look how learners can develop data sense. You will look at how data can be analysed using appropriate statistical measures and consider how sensible conclusions can be made from these statistics.

2.1 Statistical thinking and the PCAI cycle

The Open University — Wed, 01 May 2019 17:18:55 GMT

Statistical thinking involves more than just calculating averages and plotting graphs. In the English National Curriculum (2014), students are expected to ‘interpret and present data’ from 7 years of age. Students in lower secondary school, aged 11–14, are expected to ‘describe, interpret and compare’ data and to ‘describe simple mathematical relationships between two variables’.

In order for learners to develop statistical thinking, they need to be encouraged to discuss the meaning behind their calculations and to interpret measures and graphs within relevant contexts. Statistics lessons must therefore involve more than just teaching techniques and methods.

Outside a school setting, statistics are rarely calculated in isolation. Data is collected, analysed and interpreted with a purpose, to investigate something or to answer a research question.

Learners can be introduced to the concept of a statistical investigation using the PCAI cycle (Figure 6).

PCAI stands for: pose (a question), collect, analyse and interpret (the data).

Figure _unit9.2.1 Figure 6 The PCAI cycle

Example

Here is an example of a simple statistical investigation with young learners.

Pose: learners want to find out which type of pet is favoured by their classmates (Figure 7). The question that they will investigate is: ‘What is the most popular pet?’

This could be further refined as: ‘What is the most popular pet in our class?’

Figure _unit9.2.2 Figure 7 A miscellany of pets

Collect: learners will ask each of their classmates what their favourite pet is, and record the results in a tally chart, similar to the one in Figure 8.

Figure _unit9.2.3 Figure 8 A tally and frequency chart

Analyse: learners will add up the totals in their tally charts, to compare the popularity of each pet. They will then produce a bar chart to present this data visually (Figure 9).

Figure _unit9.2.4 Figure 9 A bar chart

Interpret: learners will look at their bar charts and totals from their table to decide which is the most popular pet in their class.

This is a very simple example of a statistical investigation, but the PCAI cycle can be used with learners of all ages and experiences as the question posed, data collection method, data analytics and interpretation of data can all be varied in levels of sophistication.

For instance, in the UK, older students may use a large data set from the Office for National Statistics to investigate employment levels, using box plots to compare employability in different ages groups.

2.2 Analysing data using measures of central tendency and spread

The Open University — Wed, 01 May 2019 17:18:55 GMT

The mean, median and mode are measures of central tendency. Each of these averages gives an indication of the typical or central value in the data set.

The range is a measure of spread. It tells us how much a data sample is spread out or scattered.

These measures can be used to summarise a data set and help us to make conclusions about what the data shows.

The mean

The mean is the most common measure of average. To calculate the mean, add the numbers together and divide the total by the amount of numbers:

Mean = sum of numbers ÷ amount of numbers

Activity _unit9.2.1 Activity 8 Sharing sweets fairly

Allow 5 minutes

Three friends have each been given some sweets (Figure 10).

Peter has 3 sweets, Aisha has 8 sweets and Holly has 1 sweet.

How could the sweets be shared fairly between the three friends?

Figure _unit9.2.5 Figure 10 Sharing sweets with friends

Discussion

This question is asking for the mean average. Between the 3 children there are 12 sweets. Dividing this by 3 would mean that each friend receives 4 sweets.

The mean average is 4.

Activity _unit9.2.2 Activity 9 Reflecting

Allow 5 minutes

How would you present the question above to your learners?

How would you help them to understand what the mean represents?

Discussion

Using concrete resources such as counters, cubes or coins can help learners to understand the concept of the mean average (Figure 11). They can physically move the objects to share them out evenly.

Figure _unit9.2.6 Figure 11 Finding the mean with concrete resources

This can become more challenging when learners are required to find fractions of amounts.

For example:

If four learners have 3, 5, 4 and 2 cubes respectively, what is the mean number of cubes?

3 + 5 + 4 + 2 = 14

14 ÷ 4 = 3½

IIn this example, cubes could be used to show that each learner gets 3 cubes and that there are 2 cubes left over. Depending on the age and experience of the learners, they can then record the mean as 3 remainder 2, or they can discuss how to share the remaining two cubes between four.

It can be helpful to have objects which can be split in half, when working on problems like this for the first time, including edible resources!

Another way would be to use pictorial representations of the problem (see Figures 12 to 14).

Figure _unit9.2.7 Figure 12 A diagram which represents the number of sweets each child has

Figure _unit9.2.8 Figure 13 All sweets together

Figure _unit9.2.9 Figure 14 All sweets shared equally between three children

With pictorial representations, halves, quarters and other fractions can be represented more easily.

The mode

The mode is the value that occurs most often.

The mode is the only average that can have more than one value.

When finding the mode, it helps to order the numbers first.

For example, here are the ages of a group of learners:

10, 8, 11, 9, 10, 12, 13, 11, 10, 11

Putting them in order, from smallest to largest, we have:

8, 9, 10, 10, 10, 11, 11, 11, 12, 13

There are three 10s and three 11s, so there are two modes.

We can call this data set bi-modal. The mode is 10 and 11.

The median

The median is the middle number in an ordered set.

If there are two middle numbers, the median is the mean of those two numbers.

For example, using the same ordered set of data as above, the two middle numbers are 10 and 11, so the median is 10.5 (Figure 15)

Figure _unit9.2.10 Figure 15 Calculating the median

The median is the mean of the two middle values 10 and 11. So the median is 10.5.

Teaching idea: People maths

The Open University — Wed, 01 May 2019 17:18:55 GMT

One memorable way to demonstrate the concept of the median number is using a people maths activity.

Figure _unit9.2.11 Figure 16 Lining up for a maths activity

Ask your learners to line up (Figure 16). Then ask them to get themselves into height order, from shortest to tallest.

The learners at each end of the line (the shortest and tallest learner, respectively) then need to move away from the line at the same time.

This is repeated with the new shortest and tallest learner until there is either one or two learners left. It is important that each pair leaves the line at the same time.

If there were an odd number of learners to start with, you will be left with one learner in the line who has the median height.

If there were an even number of learners to start with, the result will be two learners left in the line.

Rather than having two median heights, the next step is to find the mean average of the two middle heights. This will be the median height of the group.

The range is the difference between the highest and lowest values in a set of numbers.

To find it, subtract the lowest number in the distribution from the highest.

2.3 Comparing distributions and making sensible conclusions

The Open University — Wed, 01 May 2019 17:18:55 GMT

There are advantages and disadvantages of using different measures of spread. See the table below.

Table _unit9.2.1 Table 7 Advantages and disadvantages

Average	Advantages	Disadvantages
Mean	Takes all values from the data set into consideration	Can be skewed by very large or very small values in the data set
Median	Very large or very small values do not affect it	It can be very slow to calculate for a large data set
Mode	The only average which can be used for non-numerical data	There could be more than one mode There may be no mode It might not represent the data set

Average

Advantages

Disadvantages

Mean

Takes all values from the data set into consideration

Can be skewed by very large or very small values in the data set

Median

Very large or very small values do not affect it

It can be very slow to calculate for a large data set

Mode

The only average which can be used for non-numerical data

There could be more than one mode

There may be no mode

It might not represent the data set

When analysing data, it is important to consider which measures of spread will be most suitable.

Activity _unit9.2.3 Activity 10 Calculating measures of spread

Allow 10 minutes

Below are the English and Maths test scores for 10 school children.

Find the mean, median, mode and range of the English scores. The Maths measures have already been calculated.

Table _unit9.2.2 Table 8 Maths and English scores

Maths	77	78	76	78	78	76	80	79	78	80
English	67	73	101	68	66	85	69	86	101	64

Table _unit9.2.3 Table 9 Measures of central tendency and spread

	Maths	English
Mean	78
Median	78
Mode	78
Range	4

Discussion

Table _unit9.2.4 Table 10 Measures of central tendency and spread

	Maths	English
Mean	78	78
Median	78	71
Mode	78	101
Range	4	37

Activity _unit9.2.4 Activity 11 Reflecting

Allow 5 minutes

If you were to compare the scores in the two subjects, English and Maths, which measure of average would you use and why?

Interactive content appears here. Please visit the website to use it

Discussion

Range: the range of scores in English (37) is far greater than that in Maths (4).

Mean: the mean score in each subject is 78, which implies that the scores of the students are more-or-less identical in English and Maths. But looking at the actual scores, you can see that this is not the case.

Median: if you compare the medians (71 and 78), you might assume that the students generally scored less in English (which is partly true, but there are also some much higher scores there too).

Mode: if you just state the modal score for each subject (101 and 78), you have no information about the scores of the other students.

So, which is best? It seems that to give maximum information, a combination of the median and the range would be best.

In summary, English has a median score of 71 and a range of 37, and Maths has a median score of 78 and a range of 4.

Making sensible conclusions

It is important to be aware that the way data is sampled and analysed will affect the conclusions we make.

Calculating only one measure of central tendency may result in a very different conclusion being formed than if a different measure was calculated. Generally, using more than one average along with the range will give learners a clearer idea of the data set they are exploring.

Misleading data in the world around us

Newspapers, adverts and political campaigns often use statistics to get their point across. As we saw in the example of the English and Maths scores, one statistical measure on its own will not give us a fair understanding of the data set.

Having data sense is about being aware of how data is collected and how statistics are calculated in order to make sense of figures presented to us and to avoid being misled by campaigns published by advertising companies and political parties. Knowing how to read and understand statistics allows us to make well informed decisions.

Similarly, the way graphs and charts are presented to us can sometimes be misleading and the choice of chart or graph can lead us towards particular conclusions.

We will discuss this in section 4 of this week.

3 Charts and graphs asvisual representations of data

The Open University — Wed, 01 May 2019 17:18:55 GMT

In Section 1, you looked at how data can be collected and recorded in tables. Visual representations of data, namely graphs and charts, can provide insights which are not easily observed from the data alone.

In this section you will look at how data can be represented visually using several different charts and graphs, including:

pictograms
bar charts
line graphs
pie charts
scatter graphs.

We will also discuss when it might be appropriate to use each of these graphs and charts.

3.1 Basic charts and graphs

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will consider pictograms and bar charts as highly visual ways of displaying data.

Pictograms

The Open University — Wed, 01 May 2019 17:18:55 GMT

Pictograms use images to represent data (Figure 17). A key is needed in order for a pictogram to make sense.

Figure _unit9.3.1 Figure 17 A pictogram of cupcakes sold

Since one cupcake image represents 6 cupcakes sold, you can see that 30 cupcakes (5 × 6) were sold on Monday and 15 cupcakes (2½ × 6) were sold on Tuesday.

Bar charts

The Open University — Wed, 01 May 2019 17:18:55 GMT

Bar charts are used to represent categorical data, such as favourite colours, and discrete numerical data, such as the number of children in a group of families.

In a bar chart the height of each bar represents the frequency corresponding to each category (Figure 18).

Figure _unit9.3.2 Figure 18 A bar chart

Activity _unit9.3.1 Activity 12 Reading data from a bar chart

Allow 5 minutes

How many people prefer sci-fi films?
How many people like comedy and romance films?
How many people were asked in this survey?

Discussion

Two people prefer sci-fi films.
There are 21 (10 + 11) people who like comedy and romance films.
A total of 36 people were asked (the sum of the frequencies).

Other types of bar chart

The Open University — Wed, 01 May 2019 17:18:55 GMT

Bar charts can also be used to compare two sets of data in either dual or composite bar charts.

Figure _unit9.3.3 Figure 19 A dual bar chart

A dual bar chart shows two sets of data side by side for comparison (Figure 19).

Figure _unit9.3.4 Figure 20 A composite bar chart

A composite bar chart is useful when the two sets of data are connected, as the frequencies are combined. (Figure 20) For example, when recording the takings from two branches of a shop, a composite bar chart will show the total takings from both branches but also show the breakdown of sales from each shop.

Line graphs

Line graphs are often used to show changes or trends over time. They are produced by plotting a series of points which are joined up by a straight line.

The general shape of a line graph allows some interpretation at first glance. For example, in Figure 21, it is clear that the rainfall was highest on the third day.

Figure _unit9.3.5 Figure 21 A line graph

Activity _unit9.3.2 Activity 13 Interpreting a line graph

Allow 5 minutes

On which day was the rainfall lowest?
On which day did 7 mm of rain fall?

Discussion

Days 2 and 8 both had the lowest rainfall of 2 mm.
A total of 7 mm of rain fell on day 4.

3.2 Pie charts

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will read about using pie charts, how to draw pie charts and the important point that pie charts demonstrate the proportions of each type of data rather than the frequency of each type of data.

Understanding and interpreting pie charts

The Open University — Wed, 01 May 2019 17:18:55 GMT

Pie charts use different sized sectors of a circle to represent data.

The pie chart below represents the first instrument played by a sample of people.

Figure _unit9.3.6 Figure 22 A pie chart

Understanding this pie chart

In this example, the most common instrument is the piano. This can be seen easily as the light blue part of the pie chart, representing the piano, is much larger than any of the other sectors.

It is also straightforward to see that ¼ of the people in this sample had a guitar as their first instrument. You can see this because the angle within the dark blue section, representing guitar, is 90°.

Using this information, you know that if the total number of people in this survey was 80, there must be 20 people who play guitar (80 divided by 4).

Similarly, if you knew that 6 people play guitar, you could work out that the total number of people in the sample must be 24 (6 × 4).

Using angles in pie charts

The Open University — Wed, 01 May 2019 17:18:55 GMT

In the example above, we used the fact that ¼ of the pie represented the number of people for whom guitar was their first instrument. But it would have been more difficult to give an accurate answer if the question had asked about the number of people whose first instrument was drums.

Since pie charts use sectors of circles, we can relate the proportions of the circle, in degrees, to the proportion of the sample and use this to find out how much each sector represents (Figure 23).

Figure _unit9.3.7 Figure 23 A pie chart showing the angles for each sector

We need some more information, either the total number of participants in this survey or the value for one of the categories.

There were 48 participants in this survey. We can now calculate how many people each of the sectors of the pie chart represent.

Table _unit9.3.1 Table 11 Instrument, angle and frequency

Instrument	Angle		Frequency
Guitar	90°	90/360 × 48 =	12
Trumpet	15°	15/360 × 48 =	2
Drums	60°	60/360 × 48 =	8
Flute	45°	45/360 × 48 =	6
Piano	150°	150/360 × 48 =	20

To check for accuracy, it is important first to check that the angles sum to 360° and, second, that the total frequency sums to what you expect, in this case to 48.

Drawing pie charts

The Open University — Wed, 01 May 2019 17:18:55 GMT

Drawing pie charts accurately requires the same process in reverse (Figure 24).

Figure _unit9.3.8 Figure 24 Constructing a pie chart

Teaching idea: Do Smarties® have the answer?

The Open University — Wed, 01 May 2019 17:18:55 GMT

In Week 3, we discussed the use of concrete materials, including edible resources when appropriate.

In this classroom activity, learners are given a tube of Smarties^® (or similar sweets with a range of colours), either one each or in pairs.

The learners are then required to record the frequency of each colour of sweet in a tally and frequency chart. Learners can use the sweets to create their own visual representations of data. Bar charts work really well – see Figure 25.

Data from each individual, or pair, can then be added to a whole- class table of results. Using an Excel spreadsheet is a good idea here.

Learners can compare their frequency charts and bar graphs with the whole class results, allowing for discussions about natural variation in samples. This will lead to the understanding that larger samples will generally be more representative of the whole population.

This fun investigation allows learners to use the PCAI cycle, while thinking about how they can visually represent data.

Figure _unit9.3.9 Figure 25 Smarties^® bar chart

3.3 Scatter graphs

The Open University — Wed, 01 May 2019 17:18:55 GMT

So far this week, you have mainly focused on graphs and charts which are used to display single variables.

In this section you will consider bivariate data, which will involve looking at data for two variables at once. One example would be to look at the sales of umbrellas per day alongside the hours of rainfall per day.

Scatter graphs show the relationship between two sets of variables. By looking at a scatter graph you can see whether there is a link between two variables.

When there is a link it is called correlation.

Correlation

Scatter graphs can have positive correlation, negative correlation, or no correlation.

Positive correlation means as one variable increases, so does the other variable (Figure 26). They have a positive connection.

For example: as it snows more, sales of de-icer sprays go up.

Figure _unit9.3.10 Figure 26 A scatter graph showing a positive correlation

Negative correlation means as one variable increases, the other variable decreases (Figure 27). They have a negative connection.

For example: as the temperature increases, fewer hot chocolate products are sold.

Figure _unit9.3.11 Figure 27 A scatter graph showing a negative correlation

No correlation means there is no connection between the two variables (Figure 28).

For example: an increase in rainfall in the UK and the sales of bananas in supermarkets.

Figure _unit9.3.12 Figure 28 A scatter graph showing no correlation

Activity _unit9.3.3 Activity 14 Expected correlation

Allow 5 minutes

Would you expect there to be a positive correlation, a negative correlation, or no correlation between the following pairs of variables?

a.age and weight
b.cold drinks sales and temperature
c.height and intelligence

Discussion

a.In children, you might expect a correlation between age and weight but with adults, you would not expect a correlation.
b.Yes, in hot weather, people are more likely to buy cold drinks.
c.No, there is no relationship between height and intelligence.

Interpreting correlation

The Open University — Wed, 01 May 2019 17:18:55 GMT

Correlation shows a link between two variables. Interpreting this relationship with the context of the data is an important part of the PCAI cycle (Section 2.1). When correlation is found between two variables, some conclusions can usually be drawn.

For example, a negative correlation was found in one of (a), (b) and (c) in Activity 14. This suggests that there is a relationship between these two variables and that as one increases, the other decreases.

Thinking about the context of this example helps to ensure that a sensible conclusion is made.

3.4 Misleading correlation

The Open University — Wed, 01 May 2019 17:18:55 GMT

It is important to use data sense when interpreting relationships based on correlation.

Correlation, not causation

The Open University — Wed, 01 May 2019 17:18:55 GMT

Sometimes correlation exists between two variables but this does not automatically mean there is causation.

An example of this is that the sale of ice-creams is highest when the number of accidents in swimming pools is highest. To claim that ice- cream sales cause accidents in pools, or vice versa, would be false. In reality, both factors may have been caused by a heat wave. The heat wave is a hidden or an unseen variable, also known as a confounding variable.

Spurious correlation

The Open University — Wed, 01 May 2019 17:18:55 GMT

Sometimes a correlation can be found between two variables but there is no relationship or connection between them at all. This is known as spurious correlation or, in other words ‘false correlation’.

For example, the number of astronauts dying in spacecraft is directly correlated to seatbelt use in cars. At first glance there appears to be a link between the two variables but there is no relationship, so the correlation is spurious.

Having discussions with your learners about when correlation does and does not imply a relationship is a useful way to support the development of their statistical thinking.

4 Addressing common misconceptions with probability

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section we will discuss some of the common misconceptions that arise within the topic of probability and how you might address them in your teaching.

4.1 Equal likelihood

The Open University — Wed, 01 May 2019 17:18:55 GMT

‘All events are equally likely.’

When discussing how likely an event is to occur, a common misunderstanding is that all events must be equally likely. In some cases, for example when throwing a dice or tossing a coin, the outcomes are all equally likely. However, there are many instances when this is not the case.

Tim says, ‘Tomorrow it will either rain or not rain, so the probability that it will rain is 50:50.’

Figure _unit9.4.1 Figure 29 Child splashing in a puddle

Activity _unit9.4.1 Activity 15 Reflecting

Allow 5 minutes

What is wrong with Tim’s logic in this instance?

Discussion

Tim has identified two outcomes: rain or not rain. He has therefore deduced that the probability of rain is 0.5. While he is right that rain and not rain are mutually exclusive (i.e. cannot both occur at the same time), there are many factors involved in calculating a weather forecast. It is not usually the case that the probability of rain is exactly the same as the probability of it not raining.

4.2 Gambler’s fallacy

The Open University — Wed, 01 May 2019 17:18:55 GMT

‘Later events may be affected by or compensate for earlier ones.’

Many people hold the incorrect belief that dice or coins have memory. When playing games or placing bets, people often believe that their luck is about to change because ‘a 6 is due’ or ‘my numbers must come up as they haven’t come up yet’.

In fact, when using a dice, coin, lottery machine, etc., the probability of an event occurring is irrelevant of the previous results. For example, Chloe says:

‘I have thrown a dice 12 times and not yet got a 6. The probability of getting a six on my next throw must be more than .’

Comment

The Open University — Wed, 01 May 2019 17:18:55 GMT

Chloe believes that the events that have occurred already have some kind of effect on future events. In fact, while it might seem unlikely to throw a dice 12 times and not get a 6, the probability of getting a 6 on the next throw remains .

Figure _unit9.4.2 Figure 30 Hand holding dice

4.3 Sample space considerations

The Open University — Wed, 01 May 2019 17:18:55 GMT

‘If events are random, the results of a series of independent events are equally likely.’

For example: if you toss a coin twice, it is equally likely that you will get heads, heads as heads, tails.

Figure _unit9.4.3 Figure 31 Hand flipping coin

Comment

The Open University — Wed, 01 May 2019 17:18:55 GMT

It is important to draw a sample space diagram for multiple events as it is not necessarily the case that all outcomes are equally likely.

Tossing a coin twice is a very good example of when this is not the case.

When one coin is tossed, the outcome is either heads (H) or tails (T), so the probability of each outcome is .

When two coins are tossed, the first coin can be either H or T and the second coin can be either H or T. This results in the sample space: HH, HT, TH, TT.

This means that the probability of getting two heads (HH) is and the probability of getting one head and one tail (TH or HT) is . The two events are not equally likely.

4.4 Spinners

The Open University — Wed, 01 May 2019 17:18:55 GMT

The probability of landing on a particular colour on a spinner is determined by the number of sections rather than the size of the angles.

Figure _unit9.4.4 Figure 32 A spinner

When looking at spinners, it is important to look at the size of the sectors. In Figure 32 there are 3 possible outcomes but the angle for the green section is much larger than for the yellow section, so the probability of landing on green is much higher.

Activity _unit9.4.2 Activity 16 True or false?

Allow 5 minutes

Read each of the statements below and decide whether they are true or false. Give reasons for your decision and try to explain the misconception you think each learner has formed.

(a) I flipped a coin 3 times and got 3 heads. It is more likely to be tails than heads if I spin it again.

(b) I tossed a coin 8 times and got tails every time. That proves that this coin must be biased as half of the time I should have got heads.

(c) I roll two dice and add the results. The probability of getting a total of 6 is because there are 12 different possibilities and 6 is one of them.

(d) If six dice are thrown at the same time, I am less likely to get 1, 1, 1, 1, 1, 1 than 1, 2, 3, 4, 5, 6.

(e) I spin two coins. The probability of getting heads and tails is because I can get heads and heads, head and tails, or tails and tails.

(f) Each spinner has two sections – one black and one white. The probability of getting black is for each spinner.

Figure _unit9.4.5 Figure 33 Three spinners

Interactive content appears here. Please visit the website to use it

Discussion

(a) False.

The probability remains .

(b) False.

It could be a biased coin but the point is, this is not enough evidence to prove that it is. If it were rolled 100 times with only tails appearing, that would give a fair indication that the coin is biased. However, it is possible to get tails 8 times in a row, so it is not enough in itself to prove that the coin is biased.

First, the sample space is the totals which can be made from two dice. Totals can be made from 2 to 12 (1 is not possible with two dice). Second, the probability of each score is not equal. For instance, 7 can be made in 6 different ways: 1 + 6, 2 + 5, 3 + 4, 4 + 5, 5 + 2, 6 + 2, whereas 2 can only be made in 1 way: 1 + 1.

The probability of getting a 6 is .

(d) False.

Both outcomes are equally likely. The sample space would be all of the possible combinations. So 1, 1, 1, 1, 1, 1 is no less likely than 1, 2, 3, 4, 5, 6.

Of course, if you are not worried about the order (i.e. you can have the numbers 1 to 6 in any order) then that outcome is more likely than 1, 1, 1, 1, 1, 1 as there is only one arrangement of this but the numbers 1 to 6 can be ordered in 6 × 5 × 4 × 3 × 2 × 1 ways.

(e) False.

We discussed this example earlier. Yes, there are three outcomes if you don’t consider the order, but there are two ways to get heads, tails – HT and TH – so the probability of getting heads, tails is .

(f) False.

In the first spinner in Figure 33 the probability of landing on black is as half of the spinner is shaded black.

In the second spinner, the probability of landing on black is only .

In the third spinner the probability of landing on black is .

It is important to take the proportion into consideration, not just how many different outcomes there are.

4.5 Building misconceptions into your teaching

The Open University — Wed, 01 May 2019 17:18:55 GMT

You have considered some of the common misconceptions that arise when teaching probability in the middle years. As you teach this topic you are likely to find others.

In Week 3, where you looked at common misconceptions with proportional reasoning, we discussed the importance of tackling misconceptions directly. Due to the abstract and sometimes counter-intuitive nature of probability, it will be is important to ensure that your learners have a clear understanding of the rules and logic involved.

Using quotes like those in Activity 16 as starter activities to stimulate discussion is a good way to address misconceptions with your class.

5 Making the connections between theoretical and experimental probability

The Open University — Wed, 01 May 2019 17:18:55 GMT

In this section you will look at the connections between theoretical and experimental probability.

5.1 Experimental probability

The Open University — Wed, 01 May 2019 17:18:55 GMT

Have a go at this probability game and then reflect on what it demonstrates.

Activity _unit9.5.1 Activity 17 Horse race

Allow 10 minutes

Several people can play this game (or a whole class can play). Every person chooses a number to represent their horse. They each take it in turns to roll two dice and sum their numbers and when their number comes up they shade one box for their horse.

So if someone gets a 2 and a 3, their total is 5 and they shade one square on the 5 line. The race is won by the first horse to get to the finishing line (that is when all five boxes on their line are shaded).

When the game is finished you should have a horizontal bar chart.

Figure _unit9.5.1 Figure 33 Horse race

www.random.org/ dice/ ?num=2

If you do not have any dice, there are simulators available online, for instance:

www.transum.org/ software/ SW/ SnailRace/

Activity _unit9.5.2 Activity 18 Reflecting

Allow 5 minutes

Some questions to get you thinking about the activity.

How did your horse do?
Did anything surprise you about the results you recorded?
Was the winning horse one you might have expected to win?
Did you notice anything about horse number 1?

You will calculate the theoretical probabilities of each horse moving forward in the race later, in Section 5.3.

5.2 Relative frequency

The Open University — Wed, 01 May 2019 17:18:55 GMT

The experimental probability, or relative frequency, of an event is the ratio of the number of outcomes in which a specified event occurs to the total number of trials in an actual experiment. This is different from the theoretical probability of an event, which we will discuss in Section 5.3.

In the horse race example, the event is ‘getting a score of 8 on two dice’ (or any of the other totals) and the total number of trials is the number of times the two dice are thrown.

For example: the horse race game was played by rolling a pair of dice 100 times. This means the total number of trials is 100.

The frequencies of each total score have been recorded in the table in Figure 34. These represent the number of outcomes of each specific event (i.e. the number of times each score was rolled). The bar chart gives an overview of the distribution of the data.

Figure _unit9.5.2 Figure 34 Experimental frequency of the horse race

The relative frequency is calculated by dividing the number of times each event occurred by the total number of trials.

For example, if the event is ‘scoring an 8 when rolling two dice’, the relative frequency is

since a score of 8 was recorded 6 times and there were 100 rolls altogether.

You can see the relative frequencies of all 12 scores, shown as fractions, in Figure 35.

Figure _unit9.5.3 Figure 35 Relative frequency of the horse race

Note that the total of the relative frequencies must sum to 1, since all possibilities have been included.

We will discuss how experimental probability (relative frequency) relates to theoretical probability in Section 5.4.

5.3 Calculating theoretical probability

The Open University — Wed, 01 May 2019 17:18:55 GMT

As well as conducting your own probability experiments, you can calculate the theoretical probability of events occurring using what you know about the event and the other possible outcomes.

In order to calculate the probability of each horse moving forward, you need to understand the sample space, which includes all of the possible outcomes.

Looking at one dice is simpler. There are 6 possible outcomes: 1, 2, 3, 4, 5, 6.Assuming that you are using a dice, each event is equally likely, so the theoretical probability of getting a 6 (or any other score) on a dice is:

When looking at two dice, it gets a bit more complicated as you can get 1, 2, 3, 4, 5, 6 on each of the two dice. So you have to consider all of the possible combinations and the resulting scores.

The sample space diagram in Figure 36 shows all of the possible combinations of scores.

Note: the smallest score is 2, since it is not possible to get a score of 1 using two dice.

Figure _unit9.5.4 Figure 36 Sample space diagram for scores from two dice

From the sample space diagram, you can see that there are 36 possible outcomes when rolling two dice. The probability of each event is not 1 in36 though. Some events are more likely than others.

For example, a score of 4 can be made in three ways: 3 then 1, 2 then 2, or 1 then 3, whereas a score of 2 can only be made in one way.

Figure 37 lists the frequency of each event in the sample space. We will use this to calculate the theoretical probability.

Since there are 36 possible outcomes, the theoretical probability for each event will be:

Figure _unit9.5.5 Figure 37 Theoretical probabilities of rolling two dice