It's 50/50. The problem in your logic is, after he shows a goat, your still figuring in the door that we now know has goat as a possibility, she can no longer choose that door which brings the possible selection down to two doors and since she can only choose one, that means she is choosing one door out of two possible doors, meaning a one in two chance... It can ONLY be a two out of three chance if all three doors are unknown and she can choose two of them. I'm not trying to be a troll here, but the level of stupidity required for adults not to understand 7th grade math is astounding.

You have just proven that it is you that doesn't understand 7th grade math, and have thus been labeled a "troll."

To sum it up for you, you originally have a 1/3 chance of picking a winner, which means you have a two thirds chance of picking a loser. Odds are you picked a loser. If the host shows you the OTHER loser, then that means that it is a good bet that the third door is actually the winner, and you should switch. You would be a fool, or a troll, if you did not.