Prepare a deck of twenty cards : the Ace to Ten of Spades and the Ace to Ten of Hearts.
Ask someone to select secretly one Spade and one Heart from the deck, explaining to him/her that an Ace counts as 1.
Now instruct him/her to:
Multiply the value of the Spade by 6.
Add on the value of the Heart.
Double the results.
Subtract the value of the Spade.
Subtract the value of the Heart.
Point out to him/her that, since he/she has now subtracted everything he/she began with, he/she can hardly be giving away any secret by telling you his/her final answer.
Neverhteless, when he/she tells you his/her answer, you proceed to name both the suit and the value of the two cards he/she has chosen.
Can you work out how that detailed information can be derived from the simple answer he/she has given you?
Character is who you are when no one is looking.
If there answer is n, s is the value of the spades, and h is the value of hearts:
s = floor(n/11)
h = n % 11 (where % is the remainder of n / 11)
"In the real world, this would be a problem. But in mathematics, we can just define a place where this problem doesn't exist. So we'll go ahead and do that now..."