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two doors

topic posted Fri, January 14, 2005 - 12:53 PM by  Salix
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Here's another classic one. A variant of this one appeared in the movie Labyrinth, although she decides to guess rather than answering it. It's never really clear if she picks the right door or not...

Two guardians stand in front of two doors. One door leads to certain death, the other leads to your destination. One of the guardians always lies, and one of the guardians always tells the truth. You are only allowed to ask one question. What do you ask?
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Salix
Vermont
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  • I think this is the answer... i got this after thinking it over dinner... Haha...

    Okay, the thing is to ask any gurardian: " If i were to ask the other guardian which door leads to my destination, what would he say?"

    Lets say the honest guardian takes door 1, and the lying one takes door 2.

    If you asked the lying guardian(door 2), his answer would be that "the honest guardian would say that it the lying guardian would reply that it is the lying guardians door (door 2).

    If you ask the honest guardian, his answer would be that
    "The lying guardian would tell you it is his door(door 2)."

    So now you just choose the door which is not door 2, which is door one.

    I am not sure how to explain it well but once u get the main idea you should be able to get the answer...
    • Either way or u can ask what would the other say if i ask which one leads to my doom.
      • Well done, Paul.

        The trick with this one is to put two conditions into the question so it doesn't matter if it's the truthful or lying guardian. Your method essentially runs the question through both guardians, guaranteeing that the response will be incorrect. I don't think I've heard that particular solution before. Very clever.

        Another way is to ask "does the lying guardian guard the safe path?" or some variant. If the guardian you ask responds "no", then you take that door. If the guardian you ask responds "yes", you take the other door. Essentially the same logic applies.

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