Multiply 47 × 99 in one subtraction

Question

When a factor is a whisker below a round number, round up and correct. Solve in your head. What is 47 	imes 99? Aim: under 8 seconds.

Accepted Answer

Replace 99 with 100 and subtract one copy of 47: 47 	imes 99 = 47 	imes 100 - 47 = 4700 - 47 = 4653. So 47 	imes 99 = \mathbf{4653}. The subtraction is clean: 4700 - 47 = 4700 - 50 + 3 = 4653. There's even a digit pattern here: 47 - 1 = 46 gives the left half, and 100 - 47 = 53 gives the right half, so the answer is 46\,\,53. Takeaway. Times-99 (or 999) is "times-100 minus the number." Never long-multiply against a number one step from a power of ten.

Multiply 47 × 99 in one subtraction

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