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Direct Proportion

Direct Proportion

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

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.

Example 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?

1 kg cost £9.00: 1kg is 1000g

Work out how much 100g would cost

100g = £0.90

600g is 6 x £0.90 = £5.40

Answer: £5.40

Example 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.

Give a reason in 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.