Calculating a percentageThe following formula enables us to calculate a percentage :
In a class of 28 pupils, there are 12 boys and 16 girls. To calculate the percentage of girls in the class we take the number of girls (16), we divide by the total number of pupils (28) and we multiply by 100.
The sign % reads 'percent'. Likewise, , means that 42,9 % of the pupils in the class are boys.
Another example :
If a piece of cake weighs 400g and Mary eats 80g of it, , this means that she has eaten 20% of the cake.
Using a percentageIf in a problem we are given a percentage and are asked to calculate a value, we must write the percentage in the form of a fraction (a number divided by 100) and we must know that generally, in maths, when we have 'of', it is expressed as a multiplication. This is the case with percentages.
For example, if we know that a cake weighs 500g and that Camille ate 15% of the cake, in order to know the quantity of cake that Camille ate, we calculate :
Therefore Camille ate 75g of the cake.
Calculating a discountAn article costs 50 dollars. Its price is reduced by 20%. What is its new price?
There are two steps, first of all, you must calculate the amount of the discount and then subtract it from the former price.
1. We calculate the amount of the discount. We want to calculate 20% of 50 dollars therefore we calculate :
2. We subtract : 50 - 10 = 40 therefore the new price is 40 dollars.