11+ Maths: Writing Ratios in Simplest Form (GL & CEM style)
21 October 2026
What is a ratio
A ratio is a way of comparing two or more quantities. It shows how much of one thing there is compared to another. Ratios are often written with a colon, such as 3:2.
Why simplest form matters
Writing a ratio in its simplest form makes it easier to understand and compare. It is an essential skill for 11+ maths, as exam questions often ask for the simplest form.
How to simplify a ratio
To simplify a ratio, find the greatest common divisor (GCD) of the numbers. Then divide each part of the ratio by this number, so you keep the relationship but with smaller numbers.
Key insight: Simplifying a ratio is like simplifying a fraction. Both use the greatest common divisor to make the numbers as small as possible without changing the relationship.
Worked example
Let's simplify the ratio 18:24.
Step 1: Write the ratio as 18:24.
Step 2: Find the greatest common divisor of 18 and 24. The GCD is 6.
Step 3: Divide both numbers by 6. 18 ÷ 6 = 3, 24 ÷ 6 = 4.
Step 4: Write the simplified ratio. The answer is 3:4.
Supporting your child
Encourage your child to look for common factors and practise dividing both parts of a ratio. Remind them to always check if the numbers can be simplified further.
