Tue
Sep
28
Answer: Tuesday Teaser #22
The answer is beautifully simple. Take any 10 cards from the pile to create a new pile, but ensure that you flip over every one of them.
Why does this work? Suppose of the 10 cards chosen, x are facing up. That means that in the new pile, there’ll be 10 - x facing up (because you flipped them all over), and in the old pile there’ll also be 10 - x facing up (because you removed x of them). A very elegant solution to a problem that seems, intuitively at least, fiendishly difficult.
Of course, alternatively you could always use the existentialist’s solution: throw all the cards away and make two piles of zero :)
