To see this clearly, use an easy selection of numbers which you already know the answer to so that you can see how it works. For example, if you get 10 out of 20 on a test at school, you would use the above formula to work out what percentage this is. Given these numbers, it would look like this:

10 / 20 = 0.5

0.5 x 100 = 50

The answer is therefore 50 per cent.

A more difficult formula, works in exactly the same way. For example, if you get 321 out of 462 in a test you would do the same thing, this would look like this:

321 / 462 = 0.69

0.69 x 100 = 69 per cent

To get a percentage increase or decrease simply subtract the smaller number from the bigger number and divide that answer by the original one and multiply by 100 to get a percentage increase or decrease.

For example, to get a percentage increase from 30 to 40 you will subtract 30 from 40 and divide the answer which is 10 by the original number 30 to get .33 then multiply .33 by 100 to get a percentage increase of 33 per cent.