who tells the truth? (riddle)

[sweet william]

a traveler on a lonely road in a foreign land comes to an intersection where stands two men, one of these men is a truthful man,cannot tell a lie. the other man is a liar must lie, cannot tell the truth.

traveler wants to know which way to town, what is the one question that can be asked to know what is the right way?

(the two men look alike,no way to tell who is liar or truthful by looks) (hint! double negative)

 

[London Colin]

I know this one

Believe it or not, when I was a student we had a whole course, entitled 'Problem Solving', that largely featured classic puzzles of this kind. So I'll have to recuse myself from this one.

I would just add that when I learned the answer, being a bit of a pedant, it seemed to me there were a couple of pieces of information missing from the question ...

1. The guardians of the intersection both know everything about the situation, as you have described it.
2. They also both genuinely do know the way to town.

 

[Monkeysystem]

Which is Which?

Do we know which is the liar and which tells the truth? Because if we did this is a no brainer.

However, if we don't know which is the liar and which is the truth teller this is a more interesting puzzle.

 

[London Colin]

Do we know which is the liar and which tells the truth? Because if we did this is a no brainer.

Not only do we not know who's who, but we only get to ask one of the guardians our question. (In case you thought it was a case of comparing their responses.)

I suppose there ought to be a further rule stated: that the liar can't be evasive in his answer. Otherwise, he could fulfill his 'never tells the truth' remit, in response to most questions, with a simple 'I don't know'.

 

[sweet william]

not only do we not know who's who, but we only get to ask one of the guardians our question. (in case you thought it was a case of comparing their responses.)

i suppose there ought to be a further rule stated: That the liar can't be evasive in his answer. Otherwise, he could fulfill his 'never tells the truth' remit, in response to most questions, with a simple 'i don't know'.

you may ask both men as many questions as u wish,similtaneous or individualy. The trick is how u word question.

 

[LeftNut]

you may ask both men as many questions as u wish,similtaneous or individualy.

Based on that, I'd just hold up two fingers and ask each of them how many I was holding up. Then I'd know which man was which.

Problem is that the original riddle said one question.

 

[London Colin]

Based on that, I'd just hold up two fingers and ask each of them how many I was holding up. Then I'd know which man was which.

Problem is that the original riddle said one question.

Indeed. Having determined the liar by that cunning ruse, you could then just ask the way as your follow-up question.

Just one, carefully-crafted question, has to be enough on its own to tell you which road to take.

It is true though, I suppose, that you could ask them both, if you wished. As it happens, you don't need to. The same question will get you the information you need, whether asked of the liar or the truer.

 

[sweet william]

okay.... this is the question, that asked of both men that traveler will know the way to town.

IF I HAD ASKED YOU YESTERDAY THE WAY TO TOWN,WHAT WOULD YOU HAVE SAID?

(YESTERDAY LAIR WOULD HAVE TOLD YOU WRONG WAY,BUT TODAY HE MUST LIE ABOUT WHAT HE WOULD HAVE TOLD YOU YESTERDAY)

TRUTH SAME TODAY AS YESTERDAY,SO ANSWERS MATCH
THANKS COLIN FOR YOUR HELP , I THINK U REMEMBER THIS PUZZLE BETTER THAN I DO

 

[toolman1]

As I remember this riddle, you are only allowed to ask one question to only one person. The question you ask is:

IF I ASKED THE OTHER GUY "HOW DO I GET TO TOWN?", WHAT WOULD HE SAY?

The answer will always be the wrong road so the traveler would take the other road.

I guess this riddle has more than one answer.

 

[London Colin]

As I remember this riddle, you are only allowed to ask one question to only one person. The question you ask is:

IF I ASKED THE OTHER GUY "HOW DO I GET TO TOWN?", WHAT WOULD HE SAY?

That's the variant I know too, but William's works in much the same way.

(Actually, if you think about it, the yesterday part is really redundant. It could just as easily be: IF I ASKED YOU "HOW DO I GET TO TOWN?", WHAT WOULD YOU SAY?)

The answer will always be the wrong road so the traveler would take the other road.

... except now the answer will always be the right road. (And the truthful one doesn't actually need to know that the other is a liar.)


There's a pleasing symmetry about our version, though :-

The liar lies about the truth that the truer would tell. The truer tells the truth about the lie that the liar would tell. And the two things are the same.