Famous brain teaser

[ajohn]

So reguardless of how many doors there were before, there are now two doors. Lets call them A) and B).
So the prize is behind one of those doors, so you have 50% chance of finding the prize behind door A). If it isnt there then it is behind door B) (The other 50%)

The host opens the no win door of the the remaining two. There was a 2/3 chance that the win was in that pair when the player picked their door. 3 doors so a 1/3 chance of being the correct one. 2/3 chance the win was in the other 2 doors then. The host opens the one that isn't a win so the remaining door has a 2/3 probability of being the win.

 

[Justin Passing]

Blimey, 4 pages, when it's immediately clear to some.