Final answer:
When adding or subtracting numbers, round the result to the same number of decimal places as the number with the fewest decimal places.
Step-by-step explanation:
When adding or subtracting numbers, the rule for significant figures is to round the result to the same number of decimal places as the number with the fewest decimal places. This means that the answer should be reported to the same decimal place as the least precise measurement. For example, if you were to add 1.0023 g and 4.383 g, the result would be rounded to the nearest thousandth, resulting in 5.385 g. Similarly, if you were to subtract 421.23 g from 486 g, the result would be rounded to the nearest hundredth, resulting in 64.77 g.