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Maths

Rounding and estimating

Estimating

We can use significant figures to get an approximate answer to a problem.

We can round off all the numbers in a maths problem to 1 significant figure to make 'easier' numbers. It is often possible to do this in your head.

Question

Find a rough answer to 19.4 over 0.0437

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Answer

We first round off both numbers to 1 significant figure (s.f.):

19.4 = 20 (1 s.f.)

0.0437 = 0.04 (1 s.f.)

So we now need to make the denominator a whole number. We can do this by multiplying both 20 and 0.04 by 100. 20 over 0.04 = 20 x 100 over 0.04 x 100 = 2000 over 4

Divide everything by 4.

= 2000 over 4 = 500

The real answer to 19.4 ÷ 0.0437 = 443.9359... So this was a good estimate.

Try this one. Remember, the working you do is just as important as the answer.

Question

How would you get an approximate answer for 386062 × 0.007243?

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Answer

Did you get the answer 400000 × 0.007 = 2800?

If so, well done! You rounded off correctly and worked out the approximate answer.

Rounding to 1 s.f.

386062 = 400000

0.007243 = 0.007

So 400000 × 0.007 = 2800

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