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28 August 2014
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A builder needs to work out a quick approximate answer to this calculation:
47.9 x ((8.94+11.7)/39.04)
Find a suitable approximation for the builder.
Show clearly the method you have used.
(2 marks)

Answer and commentary

Since we are finding an approximation, the exact answer cannot matter.
This would be an easy question to find the exact answer to using a calculator, which the builder could get, if that was important. But we want a quick method to get a feel for the answer.
Normally we would work to 1 significant figure. That means only using the first digit, but keeping to the correct size.
47.9 is close to 50.
8.94 is close to 9 but 10 would be easier and close enough.
11.7 is close to 12 but again 10 is fine because we went up a bit with the 8.94, so going down a bit with the 11.7 will compensate.
39.4 is close to 40.
So, our question becomes:
(50 x ((10+10)/40))=(50 x (20/40))
Now 20/40 is the same as 1/2.
So, we have
50 x (1/2) which is 25.

This explanation is long, but look again at the calculation and see how you could have done it in your head in one go.
Answer: 25

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