A percentage is an amount out of 100

There are number of ways to calculate a percentage of a quantity.

PERCENTAGE OF A QUANTITY

Mental strategies

When working out a percentage of a quantity in your head it is a good idea to have in mind some of the equivalent values between decimals fractions and percentages.

Using these facts many percentages can be calculated mentally.

Other methods of calculation

Quite often a calculator will be used to work out percentage problems. As with any calculation you should always estimate your answer first, using mental strategies.

Most calculators have a percentage button. The key procedure will differ depending on the type of calculator. Onepopular method, which will always work with any calculator is to convert the percentage to its decimal equivalent and then multiply.

PERCENTAGE CHANGE

In some subjects percentage change has to be calculated. This can be thought of as the

e.g. Calculate the percentage profit if an item is bought for £2 and sold for £2.50. The change in price is 50p and the original cost was £2

N.B. £2 needs to be changed to 200p

It is also possible to reverse this type of calculation. In other words find the original price given the new price and the percentage change.

e.g. The price of a T.V. set is £99. If this represents a 10% increase on the original price, what was the original price.

If the price has been increased by 10% the new price represents the old price (100%) plus the change in price (10%). In other words the new pice represents 110% of the old price. By using the decimal equivalent of 110% which is 1.1, the calculation becomes:

SIMPLE INTEREST

Simple Interest can be calculated by substituting values into the formula