Example 1.4: Addition and Subtraction with Significant Figures

Rule: When we add or subtract numbers, we should round the result to the same number of decimal places as the number with the least number of decimal places (i.e., the least precise value in terms of addition and subtraction).
(a) Add 1.0023 g and 4.383 g.
(b) Subtract 421.23 g from 486 g.

Solution

\(\mathrm{sum}_{\mathrm{a}}\) \(= 1.0023\ \mathrm{g} + 4.383\ \mathrm{g}\)

\(\ \ \ =5.385\ \mathrm{g}\)


Answer is 5.385 g (round to the thousandths place; three decimal places)

\(\mathrm{diff}_{\mathrm{b}}\) \(= 486.\ \mathrm{g} - 421.23\ \mathrm{g}\)

\(\ \ \ =65.\ \mathrm{g}\)


Answer is 65 g (round to the ones place; no decimal places)