Multiply 64 × 25 in one move

Question

Twenty-five is one of the friendliest multipliers once you see why. Solve in your head. What is 64 	imes 25? Aim: under 8 seconds.

Accepted Answer

Because 25 = 	frac{100}{4}, divide by 4 and append two zeros: 64 	imes 25 = \frac{64}{4} 	imes 100 = 16 	imes 100 = 1600. So 64 	imes 25 = \mathbf{1600}. Dividing 64 by 4 is instant (16), and multiplying by 100 is just placing zeros. Equivalently, think of it as counting quarters: 64 quarters is 64/4 = 16 dollars, i.e. 1600 cents. Takeaway. Times-25 means "divide by 4, then 	imes 100." Likewise 	imes 50 is "halve, then 	imes 100." Round-number reciprocals of 100 are your friends.

Multiply 64 × 25 in one move

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