Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

That scene in that movie

  1. Sep 14, 2008 #1
    in 21, when kevin spacey plays 'the 3 doors' with the lead character in a classroom, the guy says something like:"before you opened the door, the probability of each door having a car was 33.33% but now since door 3 has a goat and i have door1, the probability of door 2 adds up to 66.66% so i will take door 2 now." shouldn't the probability have become 50% for both doors?
  2. jcsd
  3. Sep 14, 2008 #2


    User Avatar
    Science Advisor

    No! When door 1 was chosen, it had a probability of 1/3. Opening door 3 doesn't change that. Therefore door 2 has a probability of 2/3.
  4. Sep 14, 2008 #3


    User Avatar
    Science Advisor
    2017 Award

    It sounds like you're referring to the classic Monty Hall problem. The answer is indeed 2/3 but it assumes that you know the host's behavior exactly.
  5. Sep 27, 2008 #4


    User Avatar

    Actually, door (2 or 3) had 2/3 probability. Now that door 3 has 0 prob after openning, prob of door 2 becomes 2/3.
    To remove your confusion, think of 1000 doors behind one which there is a prize. You select one door and then 998 of the remaining doors are openned which are empty. Should you not then switch your choice?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook