Math Is Fun Forum

I have a friend, who is 16 and always succeeds in confusing everyone around him. i however at least attempt to untangle his insaneness. today he was talking about probability formulas and something like quantum uncertainty. the situation was; a man has a choice between three doors, an object is behind one door. the man has chosen a door, what is the probability of the object being behind his chosen door, My 14-year old, algebra one brain instantly jumps to 1/3!! but he said that the assumption was 1/2 but it was actually 2/3..... i have been pondering for the last few hours, i think i have come up with something, but i was hoping for some sort of clarification in any way, some sort of base, he said something about looking up formulas...but im lost! i would appreciate if someone would help.