Sums and Differences

For addition and subtraction, the number of significant figures can be found by visual inspection. For example, in the expression 3.4 + 0.020 + 7.31 = 10.730 (round to 10.7)= 10.7 (rounded) the second and third decimal places in the answer cannot be significant because 3.4 is uncertain in the first decimal place. Hence, the result should be rounded to 10.7.

We can generalize and say that, for addition and subtraction, the result should have the same number of decimal places as the number with the smallest number of decimal places. Note that the result contains three significant digits even though two of the numbers involved have only two significant figures.