Question

In how many ways can 4 letters be dropped in 6 mailboxes so that no two letters go in the same box

Answer 1

you can cut em in half and then drop them in the mailbox

Answer 2

If it doesn't matter which letter goes into which box then  its 6! / 4! 2!  = 15 ways
If it does matter then its 6! / 2! = 360 ways