# Hats and Derangements (VIDEO!)

A lazy teacher hands 20 graded assignments back to his 20 students – randomly. What is the probability no student receives her own paper? A hat check clerk forgets who handed in which hat and so decides to return them at random. What are the chanced no-one receives his own hat?

In this video we find a formula for the number of derangements of N objects and show the classic appearance for the number e.