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 9complements 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 reply to
comment by Charlie)
And of course you are correct. There is an error in the expression to be computed. I don't care to square 3digit numbers mentally.
A better challenge may be: Find an expression that is easily calculated mentally, but is not immediately apparent.

Posted by Jer
on 20160629 10:45:33 