Compute 640 × 99 in your head

Question

Solve in your head. What is 640 	imes 99? Aim: under 10 seconds.

Accepted Answer

Treat 99 as 100 - 1: 640 	imes 99 = 640 	imes 100 - 640. That is 64000 - 640. Subtract: 64000 - 640 = 63360. So 640 	imes 99 = 63360. Takeaway. Any 	imes 99 is "shift two zeros, then subtract one copy". The same round-and-correct idea handles 	imes 98 (subtract two copies) and 	imes 101 (add one copy), turning near-hundred factors into a single adjustment.

Compute 640 × 99 in your head

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