Fuzzy Math Fuckers

The way the math works out has to do with the fact that you have to make two selections.. so it's kind of a trick question. Yes, the second choice you have to make is 50/50, but that's not really what was being asked, I'd wager. What they're probably asking is for a composite probability between your 1/3 and 1/2 choice.

Although, I'm still not sure how they figure changing doors increases your probability of picking correctly.