Replace awkward values by nearby convenient values before calculating. An estimate tests whether an answer has a plausible size; it is not a replacement for the required accurate calculation. Example 19.7 x 4.080.198 20 x 40.2=400. A calculator result near 406 is plausible; 40.6 suggests a misplaced decimal point. Round in a way that keeps the calculation simple and reasonably close. In a difference of nearly equal numbers, aggressive rounding can destroy useful information: 5.12-5.08=0.04, while rounding both to 5 loses the difference.

Replace awkward values by nearby convenient values before calculating. An estimate tests whether an answer has a plausible size; it is not a replacement for the required accurate calculation.

\(\dfrac{19.7\times4.08}{0.198}\approx\dfrac{20\times4}{0.2}=400\). A calculator result near \(406\) is plausible; \(40.6\) suggests a misplaced decimal point.

Round in a way that keeps the calculation simple and reasonably close. In a difference of nearly equal numbers, aggressive rounding can destroy useful information: \(5.12-5.08=0.04\), while rounding both to \(5\) loses the difference.

Estimating a calculation

Estimating a calculation