# Blind unshuffle

From Math Puzzle Wiki

Imagine you are in a pitch-black room with a standard deck of playing cards. You have been told that 10 cards in the deck are face-up and the other 42 are face-down. In order to be released from the room, you must reorganize the deck of cards into two piles (without being able to see!) so that each pile contains an equal number of face-up cards. How will you do it?