Multiply 356 × 1001 in your head

Question

1001 is 1000 + 1, which makes multiplication a copy-and-place operation. Solve in your head. What is 356 	imes 1001? Aim: under 6 seconds.

Accepted Answer

Split 1001 = 1000 + 1: shift the number three places and add itself back: 356 	imes 1001 = 356000 + 356 = 356356. So 356 	imes 1001 = \mathbf{356356}. A three-digit number times 1001 just prints twice. Takeaway. Times-1001 on a three-digit "abc" yields "abcabc." The 1000+1 split makes any "10\ldots01" multiplier a shift-and-add.

Multiply 356 × 1001 in your head

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