Fork in the road

Ask either twin: Will your twin say this (pointing to either road) is the correct road?

* The truth-telling twin will say no if it is the correct road and yes if it isn't (because he'll truthfully tell you the lie that his brother will tell).
*The liar will say yes if it's the wrong road and no if it isn't (because he'll lie and tell you the opposite of whatever the truthteller would say).

If the answer is no (from either twin), take that road. If the answer is yes (from either twin), take the other road.

Jespah, I like it.

Thanks! I've heard it as a 2-doors problem (one door leads to happy stuff, the other to danger) but it's the same thing either way. :-D

thanks
when I answered it, I said..."If I were to ask you if this road would lead me home, would you answer be no?" My professor told me I was very very close, but not quite there yet. He said I need to be able to tell which one is the liar and which is the truth teller. I'll give him your answer and see what he says. I almost think he has the wrong answer because it was one told and retold, and I think it has been changed. After 3 days of not figuring it out, the student that asked him the riddle told him the answer. I'll let you know. Thanks again!

My professor accepted that answer, but he said that wasn't the answer he was looking for.....

JESPAH...i have a question for you.

How you know which says the truth?
Because in this riddle..and in all other variants is the same answer...

Assume that liar protects the true road( right road) and the truth guy the left road.

So..if i will ask the one who says the truth: this (pointing to liar road) is the correct road? will say YES..you will go...
And if you will ask the other and ask the same: this (pointing to left road) is the correct road? he will say No

BUT you don't know which one is telling the truth..
So..if you think that you ask the liar first..you will receive as answer YES...from this you understand that the right road is wrong.

And even you had the chance of asking both of them, you will not know the true answer

So..the question: Asking the other is wrong!!! because you don't know who tells the truth..pls reply

wow..now i see that this topic is very old...2003 ..hope someone can answer me!..
I was reading riddles..and i found this in many variants..But all said if i ask the truth or the liar guy...but you don't knwo which is...

Pls someone..clear me pls

Thanks

Couldn't it be something like asking A
"If I ask B if you'd say R is the correct road, and he said yes, would he be lying?"

Not sure if it works though. Need more paper to work it out.

why don't you simply as "am i alive?" and if he says "no" then it's the wrong way

The problem as stated requires you to obtain two pieces of information by asking a single question:
1) who's who
2) which way is home

The classical version of this problem only requires that you identify the way home. Asking "am I alive?" only gets you the first piece of information.

Following on from Markr:
As Markr says, you now need two pieces of information. You cannot possibly (IMHO) learn both of these answers with one question.


My argument is that if you have only one question and do not know which twin is which, then you must ask a question such that either will give an answer you can use. For this to happen - with only one question - you must get the same answer no matter which you ask.


But if you have to get the same answer from either then you cannot possibly do this AND learn of any difference. You would need to ask two questions.


As was said earlier, one of these could be 'Am I alive?' Or any other question that the liar will answer one way and the truth-teller another, and you will know which is which. Once this has been done and you know which one is truthful and which not, you can then ask the simple question, 'Which road is safe?'


Of course I have been known to be wrong!!!

I think that you need to ask each brother "Which way would your brother tell me to go?"

Because if you ask that to the twin that tells the truth, he would tell you the WRONG way to go since his brother would lie... and if you ask that to the twin that lies, it would be the wrong way. Hence, they would tell you the SAME answer but its the WRONG way, so you would need to go the opposite way the twins say.

HokieCutie: "each brother"

That's two questions.

Which way would the other twin point to home?
The truthful twin points to where the liar points, to not home...
The liar lies, and points to the way the truthful twin would not point, not home.
Wherever they point, you go the other way...This works, but it's confusing. Moreover, I don't see how you could find which twin was which...

The riddle asks for you to ask one question to the twins, it does not state that you aren't allowed to ask both twins. It is still one question that you ask.

Also, in the riddle it states that each twin is standing at each road. When you ask them the question, both will say to go one way, which you know is a lie and the wrong way. Therefore, whoever is standing at the road you are not suppose to take, then you know that twin is the liar.

lacom100 - did your professor ever tell you the correct answer?

OK, but as your solution illustrates:

there's no need to ask the other guy as the construction of the question requires the same answer from both guys.



Nowhere in the statement of the riddle does it say that the liar is standing by the road that leads to death.

That's close, but not quite right. It assumes that one twin knows what the other will say, you can't really do that and still stay within the constraints of the puzzle.

The correct answer is "If I were to ask if this road leads to home, would you say 'Yes'?"

First let's consider the road to certain death.

The liar if asked "Does this road lead to home?" would have said Yes, so his response to the actual question "If I were to ask if this road leads to home, would you say 'Yes'?" would have been No.

The Truth-teller if asked "Does this road lead to home?" would have said no, so his response to the actual question "If I were to ask if this road leads to home, would you say 'Yes'?" would have also been No.

Now let's consider the road home.

The liar if asked "Does this road lead to home?" would have said No, so his response to the actual question "If I were to ask if this road leads to home, would you say 'Yes'?" would have been Yes.

The Truth-teller if asked "Does this road lead to home?" would have said Yes, so his response to the actual question "If I were to ask if this road leads to home, would you say 'Yes'?" would have also been Yes.

So we see that that question makes both of them "tell the truth." I hope I explained that well enough to make sense.

That answers the 'normal' version of the riddle. However, this one requires you to determine two pieces of information.

a simpler answer:

Ask "Which road would your brother tell me was the right road?"
- then take the other road.