and consequently the probability it's the {2, 4} pair is 2/5, since these are the only two possibilities. in this derivation, formula_17 is the probability that the envelope pair is the pair 1 and 2, "and" envelope a happens to contain 2; formula_18 is the probability that the envelope pair is the pair 2 and 4, "and" (again) envelope a happens to contain 2. those are the only two ways that envelope a can end up containing the amount 2.