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Succeed with maths: part 1
Succeed with maths: part 1

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6.1 Everyday multiplication

Just as with addition and subtraction, there are a number of ways that you can carry out multiplication, in your head, on paper or using a calculator. You’ll probably find for more complicated examples you will need a calculator, but if you don’t have one handy remembering how to multiply on paper will be a great help. Of course, a good knowledge of the multiplication (times) tables will also be a bonus!

To make sure that you are happy with multiplying on paper have a look at a couple of examples.

Suppose that you need to work out 9 lots of £32.50 when you are out shopping.

You need to set up the calculation so that the smaller number is on the bottom, as shown below (you have to do less work then!). Working from right to left multiply each number in turn, writing the result in line with the appropriate column. If the result is greater than 9, remember you will need to carry to the next column.

multiline equation line 1 32.50 line 2 multiplication nine low line line 3 292.50 low line line 4 super two four

If you need to multiply by a number that is 10 or more, then follow the same procedure as described in the example above to set up the calculation. Then multiply each number in the top row by each digit in the lower number in turn, finally adding up the two results.

For example, when multiplying 3965 by 25, start with the 5 and then move onto the 2. But you are not really multiplying by 2 but by 20 (2 is in the 10s place). To take this into account, add a zero to the beginning of the 2nd line of working, as shown below.

multiline equation line 1 3965 line 2 multiplication multiplication 25 low line line 3 19825 line 4 super four three two line 5 79300 low line line 6 super one one one line 7 99125 low line line 8 super one one

The next activity will give you a chance to practise this. Here’s a reminder of your times tables.

Table 3 Times table
× 1 2 3 4 5 6 7 8 9 10 11 12
1 1 2 3 4 5 6 7 8 9 10 11 12
2 2 4 6 8 10 12 14 16 18 20 22 24
3 3 6 9 12 15 18 21 24 27 30 33 36
4 4 8 12 16 20 24 28 32 36 40 44 48
5 5 10 15 20 25 30 35 40 45 50 55 60
6 6 12 18 24 30 36 42 48 54 60 66 72
7 7 14 21 28 35 42 49 56 63 70 77 84
8 8 16 24 32 40 48 56 64 72 80 88 96
9 9 18 27 36 45 54 63 72 81 90 99 108
10 10 20 30 40 50 60 70 80 90 100 110 120
11 11 22 33 44 55 66 77 88 99 110 121 132
12 12 24 36 48 60 72 84 96 108 120 132 144

If you like you can download [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]   and print a version of this table.

Activity 5 Multiplication

Timing: Allow approximately 10 minutes

On a piece of paper, perform the following operations by hand.

(a) 348 × 37

Answer

multiline equation line 1 348 line 2 multiplication 37 low line line 3 2436 line 4 plus plus 10440 low line line 5 12876

Thus, the answer is 12876. (This seems reasonable since 300 × 40 = 12000.)

(b) 560 × 23

Answer

multiline equation line 1 560 line 2 multiplication 23 low line line 3 1680 line 4 plus plus 11200 low line line 5 12880

Thus, the answer is 12880. (This seems reasonable since 600 × 20 = 12000.)

You might now be wondering about multiplying by a decimal. How do you calculate something like 30 × 0.6, for example, or even 1.2 × 0.7? One way to do it is to do the calculation first without the decimal points in, and then put the decimal point back afterwards. To work out where to put the decimal point afterwards, count how many digits after the decimal point there are altogether in the numbers you started with. Then put the decimal point in the answer to give the same number of digits after the point.

For example, to calculate 30 × 0.6, you would first do the calculation without the decimal points, so 30 × 6 = 180. You now need to put the decimal point back! In the numbers you started with, there was just one digit after the decimal point (the 6 in 0.6). So you need one digit after the decimal point in the answer. Therefore the answer is 18.0

To calculate 1.2 × 0.7, you would do the calculation without the decimal points: 12 × 7 = 84. As there are a total of two digits after the point in the numbers you started with (the ‘2’ in 1.2 and the ‘7’ in 0.7) you now need to move two digits after the point in the answer. Therefore the answer is 0.84.

Now you’ve looked at multiplication, it’s time for division, a close relation of multiplication.