Mental Math · Level Up · L6
You don't always need the exact answer — you need a quick, close one, and a way to catch silly mistakes. Round each number to something friendly, compute, and you have an estimate to trust (or to check an exact answer against).
Watch the method
38 × 42: round to 40 and 40. 40 × 40 = 1600 — a quick estimate, and the exact answer (1596) should land right next to it.
Round 38 to the nearest ten.
Round 42 to the nearest ten.
Multiply the round numbers for a quick estimate.
Your turn
Use the method, then check. Stuck? Reveal the steps.
Rounding each factor to a friendly number gives a product that's near the true value. The rounding errors are small and often partly cancel. Use it to estimate, and to sanity-check exact work.
Why it works
If your 'exact' answer is far from the estimate, you made a mistake somewhere.
Estimate 23 × 19
Round
20 and 20.
Multiply
20 × 20 = 400 (exact is 437).
Estimate 38 × 42
Round
40 and 40.
Multiply
40 × 40 = 1600 (exact is 1596).
Estimate 96 × 48
Round
100 and 50.
Multiply
100 × 50 = 5000 (exact is 4608).
Thinking the estimate is the exact answer.
It's deliberately approximate. 40 × 40 = 1600 estimates 38 × 42 = 1596.
Rounding only one of the numbers.
Round both to keep it quick — and remember each rounding nudges the estimate a little.
12 problems across three tiers. Auto-graded on submit. Hints and a full walkthrough on every question. Your work is saved on this device — sign in to keep it across devices.
t₁ = 5, d = 3 and t1=5,d=3 both count. For a list of numbers, separate them with commas (e.g. 3, 10, 17).Estimate 38 × 42 (round to the nearest ten).
Estimate 23 × 19 (round to the nearest ten).
Estimate 52 × 28 (round to the nearest ten).
Estimate 61 × 39 (round to the nearest ten).
Estimate 47 × 53 (round to the nearest ten).
Estimate 78 × 22 (round to the nearest ten).
Estimate 33 × 67 (round to the nearest ten).
Estimate 89 × 41 (round to the nearest ten).
Estimate 96 × 48 (round to the nearest ten).
Estimate 71 × 69 (round to the nearest ten).
Estimate 44 × 56 (round to the nearest ten).
Estimate 88 × 33 (round to the nearest ten).