Compute 512 × 49 in your head

Question

Solve in your head. What is 512 	imes 49? Aim: under 10 seconds.

Accepted Answer

Treat 49 as 50 - 1: 512 	imes 49 = 512 	imes 50 - 512. And 	imes 50 is "half, then times 100": 512 	imes 50 = 25600. So 512 	imes 49 = 25600 - 512 = 25088. Takeaway. 	imes 49, 	imes 51, 	imes 99, 	imes 101 are all "round-and-correct" multiplications with a single clean adjustment. Combined with the halving trick for 	imes 50, factors near 50 and 100 should cost you one subtraction, never a written multiplication.

Compute 512 × 49 in your head

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