You have four addressed letters and four addressed envelopes. If you close your eyes and randomly insert letters into envelopes, what is the probability that exactly three letters will be in the right envelopes?

Originally posted by THUDandBLUNDER You have four addressed letters and four addressed envelopes. If you close your eyes and randomly insert letters into envelopes, what is the probability that exactly three letters will be in the right envelopes?

Tricky. The probability is 0. If three are correct then the fourth must be also. Of course, the probability I gave above was for randomly placing all four envelopes correctly.

Originally posted by bbarr Tricky. The probability is 0. If three are correct then the fourth must be also. Of course, the probability I gave above was for randomly placing all four envelopes correctly.

Cool! I think you're right bbarr, the answer is zero for the reason you stated. And I was going to add that THUDandBLUNDER should have stated that only one letter should go into any one envelope, but that would given to big a clue to the solution and made it too easy.

Im pretty sure it's not zero, because it is possible for it to happen. I can't remember the equation off hand, but it's not quite 4! - I'll consult my communications book -

what if i stuffed 3 letters in the correct envelopes and then stuffed the extra letter with another letter leaving an envelop empty then there could be 3 correct and 1 false

Originally posted by Otis what if i stuffed 3 letters in the correct envelopes and then stuffed the extra letter with another letter leaving an envelop empty then there could be 3 correct and 1 false

Perhaps I should have made it clear that one and only one letter goes into each envelope, in which case the right answer is zero.