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Everyday maths 2
Everyday maths 2

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8.2 Formulas in practice

You will already have come across and used formulas in your everyday life. For example, if you are trying to work out how long to cook a fresh chicken for you may have used the formula:

Time (minutes) = 15 + w divided by 500 × 25 where ‘w’ is the weight of the chicken in grams.

For example, if you wanted to cook a chicken that weighs 2500 g you would do:

Time (minutes) = 15 + 2500 divided by 500 × 25

Remembering to use BIDMAS you would then get:

Time (minutes) = 15 + 5 × 25

          = 15 + 125

          = 140 minutes

Let’s look at another worked example before you try some on your own.

The owner of a guest house receives a gas bill. It has been calculated using the formula:

Cost of gas (£) = eight times d postfix times prefix plus of times u divided by 100

Note: 8d means you do 8 × d.

Where d = number of days and u = number of units used

If she used 3500 units of gas in 90 days, how much is the bill?

In this example, d = 90 and u = 3500 so you do:

Cost of gas (£) = eight postfix times multiplication 90 plus 3500 divided by 100

         = 720 plus 3500 divided by 100

         = 4220 divided by 100

         = £42.20

Activity 19: Using formulas

  1. Fuel consumption in Europe is calculated in litres per 100 kilometres. A formula to approximate converting from miles per gallon to litres per 100 kilometres is:

    • L = 280 divided by cap m

    Where L = number of litres per 100 kilometres and M = number of miles per gallon.

    A car travels 40 miles per gallon. What is this in litres per kilometres?

  2. You can convert temperatures from degrees Fahrenheit to degrees Celsius by using the formula:

    • C = five postfix times left parenthesis cap f postfix times times negative times 32 right parenthesis divided by italic nine

    Where C = temperature in degrees Celsius and F = temperature in degrees Fahrenheit.

    If the temperature is 104 degrees Fahrenheit, what is the temperature in degrees Celsius?



    • L = 280 divided by cap mand in this case M = 40

    • L = 280 divided by 40

    • L = 7 litres per 100 kilometres

    • C = five postfix times left parenthesis cap f postfix times negative times 32 right parenthesis divided by nineand in this case F = 104

    • C = five postfix times left parenthesis 104 postfix times minus 32 right parenthesis divided by nine

    • C = five postfix times left parenthesis 72 right parenthesis divided by nine

    • C  = 360 divided by italic nine

    • C = 40 degrees Celsius

Now that you have learned all the skills that relate to the number section of this course, there is just one final thing you need to be able to do before you will be ready to complete the end-of-session quiz for numbers.

You are now proficient at carrying out lots of different calculations including working out fractions and percentages of numbers, using ratio in different contexts and using formulas.

It is fantastic that you can now do all these things, but how do you check if an answer is correct? One way you can check would be to approximate an answer to the calculation (as you did in Section 2.2). Another way to check an answer is to use the inverse (opposite) operation.


In this section you have:

  • learned about, and practised using BIDMAS – the order in which operations must be carried out
  • seen examples of formulas used in everyday life and practised using formulas to solve a problem.