Interview puzzle with a deck of cards, some cards upside-down
This is a classic question.
Split the cards into two piles. The first pile has 18 cards, the second pile has the remaining 35 cards. Say that there are $n$ upside down cards in the first pile, and therefore $18-n$ upside-down cards in the second pile.
Now turn over all the cards in the first pile.
This is an old puzzle, basically you take any set of 18 cards and turn them over. It should be the duplicate of some other question here.