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'Monkey Puzzle' printed from http://nrich.maths.org/
Three monkeys Barry, Harry and Larry met for tea in their favourite cafe, taking off their hats as they arrived.
When they left, they each put on one of the hats at random.
What is the probability that none of them left wearing the same hat as when they arrived?
If you liked this problem, here is an NRICH task
which challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.