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).

What's your secret?

In that case, I assume that the author meant (A*C)+(B*D) = A*(C+D) = A*999 = A*1000 - A, which I can do in my head!

Posted by Steve Herman on 2016-06-30 08:22:49