◄Previous: 3.3 Have I given due consideration to units of measurement?

# 3.3.1 Try some yourself

## Activity 22

A friend has been quoted a price of £25.50 per square yard for tarmac surfacing of his yard. The yard measures 6 yards by 10 feet. Here is his calculation of the total cost. What is wrong with it?

cost = £25.50 × 6 × 10 = £1530

### Answer

He has not used the same units. The figure 10 he used refers to feet, so in a calculation using yards he needs 10/3 (3 feet = 1 yard). The cost should be

## Activity 23

Another friend, Kim, wishes to reproof her tent. A litre tin of ‘proofing agent’ states that the coverage is roughly 5 square metres per litre. The manufacturer's measurements of the tent, which is a ridge tent, are: height 2 m, length 3 m. However, Kim realises that she needs the other measurements. Here is her diagram and calculation.

The problem: how many litres of proofing agent to buy to cover the tent?

Area of 2 side panels 2 × 3 × 250 = 1500

Area of end door panels together 2 × 150 + 150 = 450

Total area = 1950

Number of litres = 1950 ÷ 5 = 390

Kim thinks this is rather a lot and asks you to help. Find her errors.

### Answer

The errors are

1. Not converting the centimetres to metres.

2. The order of operations for calculating the end door panels.

Correct answer should be

Areas are:

sides 2 × 3 × 2.5 = 15 metres squared

doors 2 × (1.5 + 1.5) = 6 metres squared

Total area = 21 metres squared

Number of litres = 21 ÷ 5 = 4.2 litres

Therefore Kim needs to buy 5 litres of proofing agent.

◄Contents►Next: 3.4 Did I make a rough estimate to act as a check?

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