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 Simple number trick (Posted on 2016-06-29)
A. Have someone write down a 3 digit number.

B. You write down the same number.

C. Have them write down another 3 digit number. (Don't let them write 999.)

D. You write down the number whose digits are 9-complements to theirs. i.e. If their first digit is 7 your first digit is 2. Try not to let them notice this.

Astound them by quickly computing (A*B)+(C*D).

 Submitted by Jer Rating: 2.5000 (2 votes) Solution: (Hide) There is an error in the problem. The given expression is not easy to compute in ones head. It should have been (A*C)+(B*D) Example: A = 379 B = 379 C = 653 D = 346 The board would look something like 379 653 379 346 To which you could add symbols to make (379*653)+(379*346) Which is easily computable as 378621 The trick is that since C+D=999 and A=B all you need to compute is 999*A = 1000A - A.

 Subject Author Date Answer Math Man 2016-07-07 08:34:02 re: Likely answer Jer 2016-06-30 09:08:26 Likely answer Steve Herman 2016-06-30 08:22:49 re: comment Jer 2016-06-29 10:45:33 comment Charlie 2016-06-29 10:13:15

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