Rounding and estimation
Most real-life math does not need an exact answer — it needs a quick, sensible one. Is 48 × 21 about 100, 1,000 or 10,000? Round to 50 × 20 and you know in a second: about 1,000.
Two lessons: rounding (to any place, with the "5 or more rounds up" rule) and estimating (round each number to its first digit, then calculate). Estimation is also the best way to check an exact answer — including one from a calculator.
How to Round Numbers
Look at the digit just after the place you round to: 5 or more rounds up, 4 or less stays. 3,478 to the nearest 100: the next digit is 7, so 3,500.
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Method 2 · Level 2How to Estimate Answers
Round each number to its first digit, then calculate: 48 × 21 ≈ 50 × 20 = 1,000. Fast, and good enough to catch big mistakes.
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