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-   -   truth teller vs liar : go to Hell ! (http://www.physicsforums.com/showthread.php?t=38284)

 humanino Aug5-04 11:38 AM

truth teller vs liar : go to Hell !

This is a well-known logical problem. But I can't figure out where it comes from, because it seems really old. I love it.

This is what happening when one dies : one gets in a room with two doors. Each door is kept by a guard. One of the doors leads to Heaven, the other one leads to Hell. One of the guard always tell the truth, the other always lies. There is no correlation between lying/telling-the-truth and keeping the door to Heaven/Hell.

You are allowed to ask only one question to only one guy. You wish to go to heaven. You can manage to find a yes/no question.

Hint : you have no hope to determine which one tells the truth. Notice that it does not matter wether you've been a bad guy or a good guy. Von Neumman certainly managed to go to Heaven, even though he was a horrible human being :biggrin: :tongue2:

 Gokul43201 Aug5-04 12:42 PM

This puzzle has been posted here before...more than once !

(-1)*(1) = (1)*(-1) = -1

 jcsd Aug5-04 01:04 PM

You ask one of the guards: "are your flies undone?", when he inevitably goes to look you give him a deft judo chop to the neck renderning him unconcious. You then sweep the other guard to the floor and get him into a 'Figure of Four Leglock' and apply pressure until he squeals like a pig.

 vikasj007 Aug9-04 03:20 AM

this problem seems to be the most faviourite of all, that's why it is posted here again and again, but anyways let me give the answer and spoil the fun for those who have not seen this before.

you ask the gaurd of one of the doors," what will the other gaurd say if i asked him, where does this door lead to."
whatever the answer he gives, the opposite is the correct answer.

HOW?

figure it out yourselves.

 Nietsnie Sep9-04 04:39 PM

Quote:
 Quote by humanino You are allowed to ask only one question to only one guy. You wish to go to heaven. You can manage to find a yes/no question.
Do you mean manage to find ONLY a yes/no question(thus expecting only 2 possible answers, viz. yes or no.)?
If so, the question posted above is invalid. As the guard would have to give a question which is neither yes/no.

Otherwise the above question seems to be the right one to ask.

 Gokul43201 Sep9-04 09:03 PM

Okay, but it can be tweaked slightly to make it work.

 Tom McCurdy Sep9-04 09:21 PM

Answer: Ask if the other person would say that this is the right path?

Lets say the door is the right path
The truth person would say that the lier would say no.
The lier woud say that the truth person would say no.

Lets say the door is the wrong path.
The truth person would say the door yes.
The lier would say the truth person would say yes.

Do the opposite of what they say.

 Nietsnie Sep11-04 07:54 AM

Yeah, that seems right to me. Same answer i worked out while i was supposed to paying attention in chemistry lecture!

 Learning Curve Sep12-04 04:21 PM

Or you could just ask them what color their shirt is, and if he tells the truth go through his door. :rofl: :tongue2: :tongue:

 fizzzzzzzzzzzy Sep20-04 10:28 PM

you could shoot one down and ask the other gaurd if he is dead

 The Bob Sep21-04 03:47 AM

Quote:
 Quote by fizzzzzzzzzzzy you could shoot one down and ask the other gaurd if he is dead
I don't think the truth teller necessarily is guarding the heaven door and vice versa.

The Bob (2004 ©)

 recon Sep21-04 04:52 AM

Quote:
 Quote by The Bob I don't think the truth teller necessarily is guarding the heaven door and vice versa. The Bob (2004 ©)
fizzzzzzzzzzzy's explanation seems pretty sound to me.

 pat777 Oct5-04 08:36 AM

Ask: What would you say if I asked you, "Does this door go to heaven?"?
The Door is the way to heaven.
The liar would say that he would say yes.
The truthful person would say that he would say yes too.

The Door is not the way to heaven.
The liar would say that he would say no.
The truthful person would say he would say no too.

This is an alternative solution.

 LastOneStanding Oct17-04 07:07 PM

Quote:
 Quote by fizzzzzzzzzzzy you could shoot one down and ask the other gaurd if he is dead
But then you'd only know who's the truth teller and who's the liar...you still wouldn't know which door is the door to heaven and which door is the door to hell. And you only get one question.

 jimmy p Oct19-04 08:01 AM

Quote:
 Quote by jcsd You ask one of the guards: "are your flies undone?", when he inevitably goes to look you give him a deft judo chop to the neck renderning him unconcious. You then sweep the other guard to the floor and get him into a 'Figure of Four Leglock' and apply pressure until he squeals like a pig.
I prefer this method!! :rofl:

 Gokul43201 Oct19-04 11:00 AM

Quote:
 Quote by pat777 Ask: What would you say if I asked you, "Does this door go to heaven?"? The Door is the way to heaven. The liar would say that he would say yes. The truthful person would say that he would say yes too. The Door is not the way to heaven. The liar would say that he would say no. The truthful person would say he would say no too. This is an alternative solution.
It does not satisfy the criterion that only one person be asked.

 gerben Oct19-04 02:40 PM

Quote:
 Quote by Gokul43201 It does not satisfy the criterion that only one person be asked.
It does, no matter who you ask you always get the right answer.

 recon Oct29-04 10:56 AM

Quote:
 Quote by LastOneStanding But then you'd only know who's the truth teller and who's the liar...you still wouldn't know which door is the door to heaven and which door is the door to hell. And you only get one question.
OMG, it's a good thing I learned this before dying. Think of all the poor people who got cheated...

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