Estimate & Check an Answer
Use rounding and order of magnitude to catch arithmetic mistakes quickly.
Round before multiplying49 × 21 should be near 1,000
50 × 20 = 1,000
Why: The exact answer should be in the same rough scale: 1,029.
Check decimal position19.8 × 5.1 should be near 100
20 × 5 = 100
Why: If a calculator result says 1,009.8 or 10.098, the decimal placement is suspicious.
Reverse the operation84 ÷ 7 = 12
Check: 12 × 7 = 84
Why: Multiplication reverses division when the divisor is non-zero.
Bounds31 × 19 is between 30 × 19 and 32 × 19
570 < answer < 608
Why: The exact 589 must lie inside those bounds.