Question 318752
The discussion regarding the Monty Hall paradox or the Let's Make a Deal paradox can get very lengthy. 
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Here's an applet that discusses and lets you play and explore:
http://www.stat.sc.edu/~west/javahtml/LetsMakeaDeal.html
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If that's not sufficient, search on "Monty Hall paradox" or the "Let's Make a Deal paradox" and you'll get plenty of pages of explanation.
Good luck!