You walk into a dark room in which 100 coins are scattered around on the floor. 10 of them are heads up, 90 are tails up.
You cannot feel, see or in any other way find out on which side the coins are. How can you split the coins into two piles such that there are the same number of heads in each pile?
You split the coins into a group of ninety and a group of ten. You then flip all of the coins in the group of ten. When the lights are turned on you’ll find that there are an equal number of heads in both groups. This method will always work.
If it’s difficult to comprehend how this works, let’s look at an example. Assume that there are 3 coins that have heads in the group of ninety. So the group of ten will have 7 coins with heads and 3 coins with tails. When all the coins are flipped in the group of ten, the number of coins with heads will become three.