A man writes 5 letters, one each to A, B, C, D and E. Each letter is placed in a separate envelope and sealed. He then addressed the envelopes, at random, one each to A, B, C, D and E.
Find the probability that both of the letters to A and B are in incorrect envelopes.
The answer is given as 13/20 but I can't figure out why. Can anyone help?