To work out an approximate answer to a complex calculation we estimate.
To estimate we round the numbers in the calculation to numbers that make the calculation easier
This is usually one significant figure (but it does not have to be).
Example 1: Estimate 4.13 × 28.1⁄0.53
To estimate the answer we need to round the numbers.
We can round 4.13 to 4 (to one significant figure). Two significant figures would probably make the calculation too complex.
28.1 could be rounded to 30 (one significant figure) or 28 (two significant figures).
0.53 rounds to 0.5 (one significant figure), again two significant figures would make the calculation too complex.
4 × 30⁄0.5
We can now calculate the estimated answer:
4 × 30⁄0.5 = 120⁄0.5 = 240
Note dividing by 0.5 is the same as multiplying by 2 (0.5 goes into 120 240 times).
Example 2: Someone gets paid £18.75 per hour. They work 29 hours in a week. Work out an estimate for how much they get paid in for the week.
State whether your answer is an overestimate or an underestimate.
We can round £18.75 to £20 (one significant figure) and we can round 29 to 30 (one significant figure).
We can estimate the pay to be £20 × 30 = £600
We rounded both the hourly rate and the number of hours up, so the actual pay will be lower than £600. We call this an overestimate.
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