Estimating calculations

Sometimes it is a good idea to estimate a calculation rather than work it out exactly, if you don’t need to know the exact value. In this situation, round the numbers in the question before performing the calculation. Usually, numbers are rounded to one significant figure. The 'approximately equal to' sign, ≈, is used to show that values have been rounded.

Examples

Estimate the value of 23 \times 67.

Rounding to 1 significant figure gives: 20 \times 70 = 1,400

Therefore: 23 \times 67 \approx 1,400

Estimate: \frac{423 - 98}{16.4}

Rounding to 1 significant figure gives: \frac{400 - 100}{20} = \frac{300}{20} = \frac{30}{2} = 15

Therefore: \frac{423 - 98}{16.4} \approx 15

Question

Estimate the value of (58.4 \div 2.79) - 9.8.

Rounding to 1 significant figure gives: (60 \div 3) - 10 = (20) - 10 = 10

Therefore: (58.4 \div 2.79) - 9.8 \approx 10.

Question

Estimate \frac{68.7 - 9.9}{2.77}.

Rounding to 1 significant figure gives: \frac{70 - 10}{3} = \frac{60}{3} = 20

Therefore: \frac{68.7 - 9.9}{2.77} \approx 20.