Deriving Proportions

# Deriving Proportions

GCSE(F),

A proportion is used to describe how a part relates to a whole and is shown as

part : whole

where the first number is the selected amount, and the second number is the total amount.

For example, if there are 8 red cars in a car park and there are 42 cars altogether. The proportion is shown as

8 (red cars) : 42 (total cars); then simplify to 4:21

Make sure that you understand if the question is asking for part:part (ratio) or part:whole (proportion).

Use the unitary method to derive proportions when comparing amounts such as value-for-money items. This involves making one side of the proportion equal to 1.

## Examples

1. A road crew have relaid part of a new road with fresh tarmac. They have completed 6km. They need to complete 8km altogether. What proportion of the road has been relaid?

The proportion is 6km out of the 8km

Rewrite as 6 : 8, and simplify to 3 : 4

2. A recipe requires 100ml of cream. Some of the cream is used for the sauce, and the rest is used for a topping. If the proportion used for the sauce is 7 : 20, how much cream was used for the sauce?