There are 4 different original letters to mail to the post office box, which contains all 6 Cabinet the probability that each letter is not a matter of how to dispose of a duplicate Cabinet.
There are four different letters to the needs left a post office box, which contains all six cabinet probability that each letter has not left the cabinet over a few ways.
A different letter 4 version. Want the letter into the mailbox, which are all 6 cabinet. The probability of each of the letters don't leave again cabinet many ways.