There is a table on which 20 coins are placed. You also know that there are 10 coins with heads up, and 10 coins with tails up. You are blind folded and there is no way to determine which side is up by rubbing, etc.
Your task is to create two groups of coins, each having the same number of heads and tails. You may only move or flip coins. How can you accomplish this while being blindfolded?
Answer
Create two groups of ten coins, then flip over the coins in one of the groups.