In this section we’re going to look at two main calculations. Firstly, how to calculate the percentage of a number and secondly how to add or subtract a percentage.

A percentage is a fraction of 100. 30% (30 in each 100) as a fraction is ^{30}/_{100}.

30% as a decimal is 0.3.

Often, in real life – and maths exams – you must find a percentage of a quantity.

So let’s look at an example to illustrate this.

Say you want to find 5% of 200.

First, write 5% as a fraction which is ^{5}/_{100} or ^{1}/_{20}.

Now multiply by the quantity.

^{1}/_{20} x 200 = 10. Simple!

Secondly we’re going to look at increasing or decreasing a number by a percent.

This goes one step further. We’re going to use the example above to keep things simple.

If an exam question asked you to increase 200 by 5% you would do the calculation above, giving you an answer of 10.

The question is asking you for an increase so you add the 10 to the original amount, making the answer 210.

If an exam question asks you for a decrease in 5% you would subtract the 10 giving you an answer of 190.

Nice and straight forward I think!