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I stumbled upon a basic probability type question from the movie 21 and was hoping someone could provide me with an explanation. The question is as follows:

If i have 3 doors and behind one of the doors is a car and behind the other 2 doors is nothing and i have to choose one of the doors. In this case it doesn't really matter which door i choose because the probability of a car being behind any door is equal that is 33.33%, so say i choose door 2. Now given that i chose door 2 what if another door was added and i had an option to either leave door 2 as my choice or pick another one the answer was to pick another one to account for variability and the fact that picking between door 1,3 and 4 will increase my chances of winning.. I don't understand how the probability of 1,3 and 4 is higher given that i picked door 2 and after which another door that is door 4 was added..

Any help will be greatly appreciated!!

Thanks