Ask the wrong question and you may die

[Washington]

You're walking along a road and come to a fork. The fork branches off to 2 different towns; one town is full of liars and they will kill you if you enter their town....the other town is full of truth tellers and they will welcome you. There is a man standing in the road from one of the two towns and you must ask him one question in order to know which town to go to without being killed of course. You do not know which town he is from.

What do you ask?

Spoiler: Answer

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Spoiler: X

"Which way to your town ?"

 

[billwald]

Which road he came down?

 

[The Nihilist]

What town would someone from the other town say was the one full of liars. Then go there.

 

[Washington]

What town would someone from the other town say was the one full of liars. Then go there.

Interesting, but I think it would be a mistake
For clarification:

T = Town of truth tellers
L = Town of liars



If the person at the cross roads is not a liar (he's from town T)
He would say "the people in town T would say that town L is full of liars" because this is the truth.


If the person at the cross roads is a liar (he's from town L)
He has a real problem. To wit:

He could not say the people in town T would say those in town L are liars, because this would be the truth, and as a liar himself, he couldn't do this.

So the burden falls on what those of town L would say, which is where the problem arises.

First of all, the people in town L would actually say the people in T were liars (because this would be a lie)

But in order to lie about this, which the liar at the cross roads is obligated to do, he would have to say the people in town L would say the people in town T are not liars. However, the question was one of, "which IS," not, "which IS NOT." So the problem then is to rephrase this into a positive assertion, which I don't see as possible. Of course the implication of "The people of town L would say the people in town T are not liars" is that they themselves are liars. Which means that they would assert that "the people in town T are liars."

So, we have the person at the crossroad who is from town T asserting that those in town L are liars, AND, by reflection, we can conclude that the people of town L would assert that the people of town T are liars.

But not knowing where the person at the cross roads is really from, we can't determine which town to head for.

So, as I read all this I don't believe your question works.

 

[nadroj1985]

How about this one:

Of the two statements - ‘You are a liar’ and ‘This road leads to the town of truth-tellers,’ is one, and only one of them true?

 

[paug]

Just ask the guy where he's from. The liar will say that he's from the "truth"-town, and point you in that direction. The honest guy will tell you that he's from truth-town and point you in that direction too.

 

[Washington]

How about this one:

Of the two statements - ‘You are a liar’ and ‘This road leads to the town of truth-tellers,’ is one, and only one of them true?

If he is a liar and says "Yes" then both statements are true (he is lying)

If he is a liar and says "No" then only one of the two are true (he is lying, but which one?)

If he is truthful and says "Yes" then only one of the two are true (but which one?)

If he is truthful and says "No" then both statements are true (he is telling the truth.


So depending on where he came from:
"Yes" could mean either both or only one are true. You wouldn't know
"No" could mean either both or only one are true. You wouldn't know

Just ask the guy where he's from. The liar will say that he's from the "truth"-town, and point you in that direction. The honest guy will tell you that he's from truth-town and point you in that direction too.

Bingo! At least that's the answer I had.

You and billwald both got it.

 

[nadroj1985]

If he is a liar and says "Yes" then both statements are true (he is lying)

If he is a liar and says "No" then only one of the two are true (he is lying, but which one?)

If he is truthful and says "Yes" then only one of the two are true (but which one?)

If he is truthful and says "No" then both statements are true (he is telling the truth.


So depending on where he came from:
"Yes" could mean either both or only one are true. You wouldn't know
"No" could mean either both or only one are true. You wouldn't know

Ah, but you didn't quite think about it hard enough. Let's say the answer you receive is "No." If it's the truth-teller speaking, this MUST mean that neither statement is true; it couldn't be both, because the truth-teller could not truthfully say he is a liar -- therefore, the road would not lead to the town of the truth-tellers. If it's the liar speaking, then one and only one of the two statements is indeed true; but obviously this one statement would have to be that he is the liar; thus the road does not lead to the town of truth-tellers. So, a "no" answer gives one sufficient reason not to go down the given road.

Now let's assume the answer is "yes." If it's the truth-teller, then exactly one of the statements is indeed true, and it cannot be the former, since the truth-teller is in fact not a liar. That would mean that the road leads to the truth-tellers' town. If it's the liar, then a yes answer implies that either neither statement is true, or both are. But he is in fact a liar, so it's not the case that neither is true. Therefore both of them are, and a "yes" answer gives one sufficient reason to take the given road.

Mine's a lot more complicated than yours, but it works, and it's more fun to think about

 

[billwald]

Ask which road he came down. If he lies, he says the road on the north because he is a liar from the south. If he tells the truth, he says the road on the north because he came from the truth town on the north. And vice versa thus each person gives a useful answer.

 

[pinqy]

"If I ask the other man which road to take to go to T, what will he say?"
The man from T will point to L (because that's what the liar would say).
The man from L will point to L (because that is not what the truth-teller would say).
Take the other road.

 

[keith99]

Just ask the guy where he's from. The liar will say that he's from the "truth"-town, and point you in that direction. The honest guy will tell you that he's from truth-town and point you in that direction too.

Good variation of the classic. Of course you have to be sure he points.

The classic riddle is just truth teller vrs liar. and the question is.

If I were to ask you if you were a liar (or truth teller) what answer would you give me.

EDIT:

I'd slightly modify your question. He could give a verbal answer with nothing more.

Instead ask:

Which road leads to your town.

Questions asking which road did you take are apt to work, but only apt. A liar could point up the road you had come by! Unless of course he had gone a bit up that road, in which case he could pick either of the two roads to the two towns.