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Use a Calculator to Calculate Reverse Percentages

In this worksheet, students will use a calculator to find an original quantity when they are given a percentage of that amount.

'Use a Calculator to Calculate Reverse Percentages' worksheet

Key stage:  KS 3

Year:  Year 8 Maths worksheets

Curriculum topic:   Number

Curriculum subtopic:   Interpret Fractions/Percentages as Operators

Difficulty level:  

Worksheet Overview

If 73% of a quantity is 328.5, what amount did we start with?


A bar model showing 73% equal to 328.5, with 100% unknown


In other words, we are trying to find what 100% is.


To do this, we need a calculator.


purple calculator

Firstly, we can turn 73% into a decimal by doing 73 ÷ 100 = 0.73

Next, because we want to go back to find what the original quantity was, we would do:

328.5 ÷ 0.73 = 450



73% of 450 = 450 x 0.73 = 328.5 

a tick mark

so we are right! 


Notice how our original quantity is bigger than the 73%? 

This makes sense, since we are finding what 100% is!


Example 1

Use a calculator to find 100%, given that 50% is 250.


Answer 1

First, find our multiplier by dividing the percentage by 100.

50 ÷ 100 = 0.5

On your calculator, type:

250 ÷ 0.5 = 500

So 100% of the quantity must be 500.


Example 2

If 80% of a number is 42, what is 100%?


Answer 2

80 ÷ 100 = 0.8

42 ÷ 0.8 = 52.5


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