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Lying about truth

  1. May 20, 2004 #1
    I suppose this one should not be very difficult for all you smart people out there, but anyways, let's see.

    This is about two brothers who together, run a phone booth at an intersection of two highways, one going to city A and the other going to city B. It is known about the brothers that the elder one always speaks the truth while the younger one always lies about anything you might ask him.

    A man going to City A came upto that intersection and now is not sure which of the two ways to take. He is aware of the strange habit of the brothers at the phone booth, and sees that one of them is at the booth. His problem is that he does know which of the brothers is at the shop at that moment.

    In order to ask about his way, he can just ask one question to that man. Find out which is that one question that he should ask so that he is certian to go the correct way.

    and remember, he can ask only one question.
  2. jcsd
  3. May 20, 2004 #2
    If I asked your brother if this is the road to city A, what would he answer?
  4. May 20, 2004 #3
    If you ask him if he has a brother, then you can tell who it is. The person who always lies will have to say no. The person who doesn't will say yes.
  5. May 20, 2004 #4


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    You ask, while imitating Sam Jackson, "Which way to City A. If you're lying to me, I'm gonna come back and kick you're ass. I don't give a damn about no logic puzzles! What kinda jackass brothers always lie while the other tells the truth! DON'T ANSWER THAT! It was rhetorical. Don't they teach you rhetoric in logic puzzle land, you lamea** m***** f*****!"

  6. May 20, 2004 #5


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    (-1)*(1) = (1)*(-1) = -1
  7. May 21, 2004 #6
    suppose it is true, the road is going to city A. If the other brother whom I would have asked, was the lying one, he would have said "no" and the brother speaking the thruth to me would tell me that honestly: "He would have said: No"

    Now, if the other brother was the honest one, he would have answered "yes", so the lying brother would invert that and say "No" as well

    So in both cases: No, this is indeed the road to city A
    And likewise the other case: Yes, this is not the road to city A.
  8. May 23, 2004 #7
    You'd ask the gentleman: "If i asked your brother about the way to city A, which road he'd give me?" and then go to the other road.
  9. May 25, 2004 #8
    andre here has given the correct answer with the correct explanation.
    GREAT WORK!!!!
    i hope next time i can come up with something bit more difficult so that it takes a bit more of your time and thought.
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