Imagine that I toss two coins. If at least one of them comes up heads, I show it to you. What is the probability that the other one is also a head?
If you have not tried to solve it, have a go now. For everyone else the answer is after the break.
There are 3 possible outcomes:
Therefore there is a one in three chance that the other coin is also a head.
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.