There are `n` letters and `n` addressed envelopes. Find the probability that all the letters are not kept in the right envelope.

Answer

`1 - (1)/(n!)`

Explanation

Required probability is 1 - P(all letters in right envelope) = 1 - 1/n!
(As there are total number of n! ways in which letters can take envelopes and just one way in which they have corresponding envelopes.)