How_To_Be_Good_At_Math

68 NUMBERS •^ PERCENTAGE CHANGES

Percentage changes

We can use a percentage to describe the size of a change in a

number or a measurement. We might also want to work out how

much an actual value has increased or decreased when we

already know how much it has changed as a percentage.

Calculating a percentage increase

12g = 20% of 60g 2g = 25% of 8g

12 ÷ 60 = 0.2

The amount

of the change

The original

amount

This snack bar used to weigh 60g but it’s now

12g heavier. What is the percentage increase

in the bar’s weight?

Then we multiply the result by 100. So, we

need to work out 0.2 × 100. The answer is 20.

Calculating a percentage decrease

2 ÷ 8 = 0.25

Here’s another snack bar. It used to contain 8g

of sugar. To make it healthier, it’s now made

with 2g less sugar. Let’s work out how much the

amount of sugar has decreased as a percentage.

To turn this result into a percentage, we just

multiply 0.25 by 100, giving us the answer 25.

Divide the size of

the change by the

original amount

12g =? % of 60g 2g =? % of 8g

First, we divide the increase in weight

by the original weight. This is 12 ÷ 60.

The answer is 0.2.

The first step is to divide the decrease in the

amount of sugar by the original amount.

This is 2 ÷ 8. The answer is 0.25.

0.2 × 100 = 20 0.25 × 100 = 25

This means the new bar weighs 20% more

than it did before.

This means the bar now has 25% less sugar.

12g

extra

less

SNACK BAR sugar

SNACK BAR

068-069_Percentage_increase.indd 68 29/02/2016 14:10