imagine what might be in the first envelope. a sensible strategy would certainly be to swap when the first envelope contains 1, as the other must then contain 2. suppose on the other hand the first envelope contains 2. in that case there are two possibilities: the envelope pair in front of us is either {1, 2} or {2, 4}. all other pairs are impossible. the conditional probability that we are dealing with the {1, 2} pair, given that the first envelope contains 2, is