Compute 104 × 75 in your head

Question

Solve in your head, no paper. What is 104 	imes 75? Aim: under 10 seconds.

Accepted Answer

Round 104 down to 100, then add back the overshoot: 104 	imes 75 = 100 	imes 75 + 4 	imes 75 = 7500 + 300 = 7800. Both pieces are trivial: 100 	imes 75 = 7500 and 4 	imes 75 = 300, sum 7800. Takeaway. Round-and-correct works in both directions: for a factor just above a round number, drop to the round number and add the correction, a 	imes b = (a - d)	imes b + d 	imes b. The sign of the correction just follows which way you rounded.

Compute 104 × 75 in your head

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