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

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3 Writing a decimal as a percentage

Because you divide a percentage by 100 to convert it into a decimal, you may assume that in order to change a decimal to a percentage, you must multiply by 100 (because multiplication undoes division). And you would be correct to assume this! A quick way to do this is to move the decimal point two places to the right.

For example, to change 0.31 into a percentage you would multiply by 100 to get 31%. Equally, 0.012 would become 1.2% after moving the decimal point two places to the right.

To try it for yourself, use a calculator to work out 0.005 x 100. The answer should be 0.5 (that is, 0.5 per cent), the same as moving the decimal point two places to the right, as expected.

Let’s take a closer look at this to make sure that this makes mathematical sense. ‘Per cent’ means ‘over 100’, so an equivalent way of expressing ‘100 per cent’ is 100 divided by 100, which is the number 1. Multiplying by 100 per cent does not change the value of a number; it just makes it look different.

If you like, you can show the conversion with one or both of the following steps: zero .723 equals zero .723 multiplication 100 percent equals left parenthesis 0.723 multiplication 100 right parenthesis percent equals 72.3 percent.

Now you need to look at writing fractions as a percentage. This can be useful, because many people will have a much better understanding of what a percentage means rather than a fraction. For example, you could say that three divided by eight, or 37.5 per cent, of the population prefers vanilla ice cream. For most people, the percentage would mean more.