Theory
A ratio compares amounts. The order matters: red:blue is not the same as blue:red unless the amounts are equal.
Ratios must compare the same type of quantity. If the question gives centimetres and metres, convert to the same unit before simplifying.
To simplify a ratio, divide every part by the highest common factor. Each part must be divided by the same number.
A simplified ratio can still have more than two parts, such as 2:3:5. The same rule applies to every part.
For Edexcel marks, show the unit conversion and the common factor when the numbers are not already easy.
Key ruleSimplify a ratio by dividing every part by the same highest common factor.