Multiply 63 × 99 with a digit pattern

Question

Times-99 can be done by subtraction, or by a two-part digit pattern worth knowing. Solve in your head. What is 63 	imes 99? Aim: under 8 seconds.

Accepted Answer

Subtraction route: 63 	imes 100 - 63 = 6300 - 63 = 6237. Digit pattern: the front is 63 - 1 = 62, and the back is 100 - 63 = 37, so the answer is 62\,\,37: 63 	imes 99 = 6237. So 63 	imes 99 = \mathbf{6237}. Both routes agree. Takeaway. For a two-digit n, n 	imes 99 = (n-1)\,\,(100 - n). Subtract 1 for the left block, take the complement to 100 for the right block.

Multiply 63 × 99 with a digit pattern

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