Use estimation to check reasonableness

Estimation involves rounding whole numbers to the nearest ten or hundred, using compatible numbers, or choosing benchmark quantities to predict reasonable sums, differences, products, and quotients in context. Comparing an exact result with an estimate reveals errors such as misplaced digits, incorrect operation choice, or an unreasonable answer, while recognizing that an estimate need not equal the exact value; the focus is on whole-number calculations rather than decimals, formal error bounds, or generalized estimation methods.