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?

Correct!  I confused myself with the variables, which the original source did not have.   I assume it will be simpler with students on a board.

Posted by Jer on 2016-06-30 09:08:26