Compute 98 × 87 in your head

Question

Solve in your head, no paper. What is 98 	imes 87? Aim: under 10 seconds.

Accepted Answer

Round 98 up to 100, then subtract the overshoot: 98 	imes 87 = 100 	imes 87 - 2 	imes 87 = 8700 - 174 = 8526. The subtraction is the only real work: 8700 - 174 = 8700 - 200 + 26 = 8526. Takeaway. Round-and-correct: replace an awkward factor with the nearest round number and subtract (or add) the correction, a 	imes b = (a \pm d)	imes b \mp d 	imes b. Any factor within a few units of 100, 50, or 1000 should trigger this automatically, never long-multiply a 98.

Compute 98 × 87 in your head

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