Multiply 67 × 9 by subtracting, not multiplying

Question

The nine times table is really the ten times table with one subtraction. Solve in your head. What is 67 	imes 9? Aim: under 5 seconds.

Accepted Answer

Since 9 = 10 - 1, take ten of them and remove one: 67 	imes 9 = 67 	imes 10 - 67 = 670 - 67 = 603. So 67 	imes 9 = \mathbf{603}. The subtraction is easy: 670 - 67 = 670 - 70 + 3 = 603. This beats grinding out 7 	imes 9 = 63 and 60 	imes 9 = 540 and adding, one subtraction instead of a carry-laden addition. Takeaway. Times-9 is "times-10 minus the number." The same round-and-correct idea handles times-8 (times-10 minus twice), times-99, and so on.

Multiply 67 × 9 by subtracting, not multiplying

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