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?

And of course you are correct.  There is an error in the expression to be computed.  I don't care to square 3-digit numbers mentally.

A better challenge may be:  Find an expression that is easily calculated mentally, but is not immediately apparent.

Posted by Jer on 2016-06-29 10:45:33