Significant Figures in Addition and Subtraction
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When measured numbers are added or subtracted, the result keeps only as many decimal places as the least precise value contributes.
Adding 1.0023 g and 4.383 g gives 5.3853 g on a calculator, but since 4.383 g only reaches the thousandths place, the answer rounds to 5.385 g. Subtracting 421.23 g from 486 g gives 64.77 g on a calculator, but since 486 g has no decimal places at all, the answer rounds to 65 g.
Students often apply the multiplication rule here by mistake, counting total significant figures instead of decimal places — addition and subtraction care only about where the decimal places end, not how many significant figures each number has, and an exact number in the sum never limits the rounding.