Estimating is used to quickly obtain an approximate answer to a calculation.
Round each number in the calculation to 1 significant figure, then perform the calculation.
For example, estimate 379 x 415:
379, to 1 significant figure, is 400;
415, to 1 significant figure, is also 400;
400 x 400 = 160 000.
Compare the estimated answer 160 000 with the actual answer 379 x 415 = 157 285. Note that the number of digits in each answer is the same, and that the answers are reasonably close to each other.
For numbers that are a single digit: e.g. 8, do not round the digit.
Sometimes it is easier to identify simpler calculations: `frac(1800)(6)` is simpler to treat as 1800 ÷ 6, rather than round 1800 to 2000 and then to divide 2000 by 6.
Note that the sign ≈ means approximate: the value on one side of this sign is approximately equal to the value on the other side of the sign.
Holly is buying some clothes in a shop. She is buying four items at £3.56, £42.49, £18.99 and £0.99. Estimate how much she is spending.
Rounding to one significant figure: 4 + 40 + 20 + 1 = 65.
The accurate answer is £66.03.
Note that as the question asked for an estimate, an answer of £66.03 would be marked as wrong.
Answer: £65
Adam has ended up with the following calculation: `frac(205 times 89)(3244)` Estimate the answer he should expect.
`frac(205 times 89)(3244)` ≈ `frac(200 times 90)(3000)`
= `frac(18000)(3000)`
= `frac(18)(3) `
= 6
The accurate answer is 5.624 (3dp).
Answer: 6