Here is a puzzle that at first sight seems completely impossible, but it actually has a simple and elegant solution. It's just that the solution is not easy to see.
You are sitting by a table wearing a blindfold, and you are told that on the table are a large number of one pound coins, perhaps hundreds. You are also told that exactly 20 of these coins are tails up, and all the rest of them are heads up.
You can touch the coins, but it is not possible to determine which side of a coin is heads and which side is tails. You can move the coins around as much as you want, and you can turn over as many coins as you want. But you will remain blindfolded throughout the task.
Your problem is to put the coins into two piles with both piles containing exactly the same number of coins that are tails up. The number of coins in the two piles that are heads up is irrelevant.
Answers at 9.00 on Monday