Direct Proportion

Direct Proportion

GCSE(F), GCSE(H),

Direct Proportion relates a part to a whole, no matter how large the whole is.

For example, if one child in 10 is left-handed, then for every 200 children, 20 would be left-handed (multiplying both numbers by 20).

When two quantities are in direct proportion, then if one of the values is zero, the other value must also be zero.

A graph of the proportion will pass through the origin as a result.

Examples

1. The price, when buying loose wood nails, is based on the weight. If 1kg of nails cost £9.00, how much will 600g cost?

Answer: £5.40

1 kg cost £9.00: 1kg is 1000g

Work out how much 100g would cost

Divide 1000 by 10 to get 100g, and divide the amount by 10

100g = £0.90

600g is 6 x £0.90 = £5.40

2. The cost of hiring a car is £20, plus a further £12.50 per day. Is the cost of hire in direct proportion? Explain your answer.

Answer: No. The initial hire of £20 is not related to the number of days hire: to be in direct proportion, the cost of hire would have to be zero when the number of days is also zero.