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by polishdude20 on December 18th, 2006

polishdude20

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Riddle:A man is sentenced to death but he gets to choose the life door or the death door. there are only two doors and they look exactly the same. there are two guards, one tells the truth always and one always lies.you can ask 1 question what is it?

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  • by Anonymous on January 14th, 2007

    Anonymous

      I ask one of the guards the following question:  “If I were to ask the other guard, which exit leads to escape, what would he tell me?”  Whichever exit is indicated by the answer that I receive, I then take the other exit.

      It does not matter whether I ask the guard that tells the truth, or the guard that lies.  I'll get the same answer either way.

      Let us suppose that we recognize both of the guards.  One guard is ex-President Bill Clinton, who we know is totally incapable of ever telling the truth about any subject.  The other is George Washington, who “cannot tell a lie”.

      Now let us suppose that exit A leads to escape, and exit B leads to certain death.

      If I pose the above question to Washington, then he knows that Clinton would lie to me and advise me to take exit B, so that is what Washington would tell me.

      If I pose that question to Clinton, then Clinton knows that Washington would truthfully advise me to take exit A; so Clinton will lie to me and tell me that Washington would advise me to take exit B.

      Either way I get the same answer.  From Washington, a truthful account of the lie that Clinton would tell, or else from Clinton, a lie about the truth that Washington would tell.  Either way, I get an answer that I know is wrong, so I would do the opposite of what it indicates.

    -------------
    [Added c. 14 January 2007, 06:30 PST]

      I don't see how I can explain it any more clearly than I did above, but it seems that several people just don't get it.  Let me see if more detail will help.

      The riddle presumes, of course, that you have no way of knowing which guard tells the truth, and which guard lies.  In the above examples, I assigned the guard roles to a known famous truth-teller and a known famous liar, so that I could show the dynamics.

      Let's stick with all the above assumptions.  The exit to freedom is A, and the exit to death is B.  George Washington will tell you the truth, and Bill Clinton will tell you a lie.

      What happens if you ask either one directly, which door leads to freedom?

      Washington will tell you the truth.  He will tell you that door A leads to freedom.  If you take his advice, you will escape.

      Clinton will lie to you.  He will tell you that door B leads to freedom.  If you take his advice, you will die.

      Now, as the riddle stipulates that you do not know who is the liar, and who will tell the truth; you can't get a useful answer by asking the question directly.  You'll either be told that door A is the one that leads to freedom, or else you'll be told that door B leads to freedom.  Without knowing whether you got that answer from the liar or the truth-teller, you cannot know whether to do as advised, or to take the other door.

      Hence the more complex question:  “If I were to ask the other guard, which exit leads to escape, what would he tell me?”

      Now, you're not directly asking which door to take.  You're asking one guard to tell you what the other guard would tell you.

      Washington knows that if asked directly, Clinton would lie, so Washington would truthfully tell you that Clinton would say that door B leads to freedom.  Of course, this is the wrong answer; as door B is the one that leads to death.

      Clinton knows that if asked directly, Washington would tell you to take door A.  But Clinton is a liar, so he will tell you that Washington would advise you to take door B.  Again, you get the wrong answer.

      Either way, you get an answer that is wrong.  You either get from Washington a truthful account of the lie that Clinton would tell you, or else you get from Clinton a lie about the truth that Washington would tell you.  Either way, you need to to the opposite of what the answer you get indicates.

    Comments

    • Elementary, my dear Watson! ;)

      nurse7263

      by nurse7263 on January 14th, 2007

    • Once again, Bob takes a fairly innocuous question, with a rather simple answer and turns it into a statement about his political beliefs. Bob, you have once again proven to me why I find you to be one of the most offensive people on AB. Just so you know, I have flagged this answer as offensive. It is not because of MY political beliefs, but because I find it offensive for you to use ANY president (current or otherwise) as an example like this.

      ptrask

      by ptrask on January 14th, 2007

    • Well, yes, if you're of a mindset to look for offense where there is none, then yes, that's how you'll see it.  I was simply using the best example I knew of someone who was famous for being honest, and the best example I knew of of someone who was famous for being a great liar.  That both examples I came up with were Presidents is beside the point, as are any political views that either might have represented.

      Anonymous

      by Anonymous on January 14th, 2007

    • ptrask..his logic is flawed as well, check the links out to my answer. Same question, just different players.

      scubabob

      by scubabob on January 14th, 2007

    • You're forgetting the flippin liar is forced to tell the truth by lying...jeezus. This is a variation of the old Kights and Knaves that's been around for 1000 years. Look it up. I'm done.

      scubabob

      by scubabob on January 14th, 2007

    • No, the liar is not being forced to tell the truth.  If you ask this question to the liar, he will lie to you about what the truth-teller would say.  Since the truth-teller would tell the truth, the liar's lie about it is not the truth.  The door that the truth-teller would advise you to take is the correct one, so the door that the liar would say the truth teller would advise you to take would be the wrong one.

      Anonymous

      by Anonymous on January 14th, 2007

    • scubabob, you should understand something about my brother. In a previous career, he was a computer programer. In order to write computer programs you need to have a very good understanding of logic. He has correctly explained the logic behind his response. Yours is the one that is flawed. Sorry, but it is true.

      Glenn Blaylock

      by Glenn Blaylock on January 14th, 2007

    • This made perfect sense to me the first time I read it and I needed no further explanation.

      muguet

      by muguet on January 14th, 2007

    • Its a standard logic puzzle (was it originially a King Solomon type question). Answered well first time round.

      cambsman

      by cambsman on January 14th, 2007

    • i got it the first time. :D good way of wording it!

      Ash

      by Ash on January 6th, 2008

    • bob, Scubabobs answer is wrong. on the webpage they directed you to (http://www.woodlands-junior.kent.sch.uk/fun.html)
      the answer is to go out the other door, like you said. so technically they just proved themselves wrong. look at the screen shot in my answer.

      Ash

      by Ash on January 6th, 2008

    • Wow your really clever.
      I get it

      SarahEatWorld

      by SarahEatWorld on August 23rd, 2008

    • Hey Ash...go thru that door.;)

      scubabob

      by scubabob on August 24th, 2008

    • Sure thing. At least its the right door. :)

      Ash

      by Ash on August 25th, 2008

    • Haha!! It actually took me 5 mins to understand Ash's answer.. Great job Ash.. that question was asked to me a year ago and I just found the answer.. Thanks!

      mastikulangan

      by mastikulangan on April 17th, 2009

    • Thanks. I didnt actually answer the question... Just investigated it :)

      Ash

      by Ash on April 24th, 2009

    • that answer was really obv. he just used big words, to make himself look clever. hahahahhaa.

      SarahEatWorld

      by SarahEatWorld on April 24th, 2009

    • I think it is more the fact that there are many conflicting answers, and nobody understood why this one was right. :)

      Ash

      by Ash on April 25th, 2009

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You're reading Riddle:A man is sentenced to death but he gets to choose the life door or the death door. there are only two doors and they look exactly the same. there are two guards, one tells the truth always and one always lies.you can ask 1 question what is it?

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