Using a Calculator

Number

Round once, at the end

Values are never rounded partway through a calculation: the full display is carried forward and only the final answer is rounded
Rounding early shifts the final figure, and the more steps that follow, the larger the shift
Where a value is needed again later, recall it from the calculator's memory rather than retyping a rounded version

Exam tip

Work to the full display and round only the final line. Then read the display for its context: money goes to two decimal places, and a decimal part of an hour has to become minutes. Where an exact answer is asked for, leave the fraction, root or multiple of π as it stands.

Worked example

The cost of rounding too early

Work out 2/3 of 285, first exactly and then using 0.67 in place of the fraction. Compare the results.

Solution:

Keeping the fraction exact: 285 × 2 ÷ 3 = 190
Replacing 2/3 with the rounded 0.67: 285 × 0.67 = 190.95
The rounded version is out by almost a whole unit, from a rounding that looked harmless
Entering the fraction itself, rather than a decimal approximation of it, avoids this entirely