White paint costs £2.80 per litre. Blue paint costs £3.50 per litre. White paint and blue paint are mixed in the ratio 3 : 2. Work out the cost of 18 litres of the mixture.

The ratio given tells us that our paint mixture of 18 litres consists of 3 + 2 = 5 parts.

Each part therefore consists of 18/5 = 3.6 litres of paint.

If we need 3 parts white paint, thats: 3 (parts) x 2.80 (£/litre) x 3.6 (litres/part) = £30.24 *

If we need 2 parts blue paint, thats: 2 (parts) x 3.50 (£/litre) x 3.6 (litres/part) = £25.20 *

Therefore the total cost of 18 litres is: 30.24 + 25.2 = £55.44

• Notice how the units cancel out here, so you know what you end up with is a cost!