## Classic brain teaser: a fork in the road and truth-tellers

You are traveling down a street and come to a fork in the road. Two guys are standing there, and one of them will always tell you a lie, and one of them will always tell you the truth.

Unfortunately, you don’t know which guy is which, and only one of the two roads leads to the town you need to get to.

Assuming both guys know which road leads to the town, what one question can you ask to find out which path to take?

As always, will try to unmask answers in a day or 2…

Joe| Guest March 8th, 2007 - 12:48 amPick a road and ask, “Will the other guy tell me that this is the right road to take to get to town?” If it’s the right road and you ask the truth teller, he’ll know that the liar will say no, and thus say no. If you ask the liar, he’ll know that the truth teller would says yes, so he’ll lie and say no. If the road was the wrong road to take, the truth teller will know that the liar would says yes, and thus say yes. If you ask the liar, he’ll know that the truth teller would say no, so he’ll lie and say yes.

jason| Guest March 8th, 2007 - 11:44 ami would ask how much farther down the left side is the town i want to go to. But, if by using the wording of the question, if they only “tell” you options, then ask them to point you in the proper direction.

Chantelle| Guest March 8th, 2007 - 2:14 pmFor this fork on the road brain teaser, I would ask both of the guys what road to take, in regards to both of the arrows on the sign. if on road does not get to the desired destination, typically you would turn around and come back the other direction then take the other or opposite turnoff (if the fork has not already punctured the tire rubber).

andrew| Guest March 8th, 2007 - 4:22 pmAsk either one what the other would answer, and go the other way

Mike| Guest March 8th, 2007 - 10:51 pmThis is a real hoary one – I remember it from when I was a kid. If you ask either what the other one will say, you will always get the opposite of the truth (one will lie about the truth, the other will tell the truth about the lie).

If you want to be tricky and not have to reverse the answer in your head, you may always ask: “If I asked the other guy which road leads to town, what is the opposite of what he would say?”

Daniel Gwozdz| Guest March 9th, 2007 - 1:39 am“If I were to ask you which road were the right one, what would be your reply?”

g| Guest March 12th, 2007 - 1:22 pmI think that I would ask one of the gentlemen what is your sex (gender)? His answer would direct me to my next question. If he told me the truth, (assuming that I’m allowed to ask the other guy a question too.) I would know that the other guy is lying and choose the other road.

My other comment is a point of clarity, are we allowed to ask each man a question, ask only one man, one question, both men several questions?

Many thanks

RK| Profile March 12th, 2007 - 7:26 pmHello g, to clarify, what one question can you ask to get the answer? This one question has to give you the right answer no matter which man you ask… hope that helps,

Thanks

Daniel Gwozdz| Guest March 13th, 2007 - 10:53 amWell, in my question, if you ask the liar, the outcome will be as follows:

If the liar is asked which side is the correct way, he’s answer the wrong way.

However, if asked how he’d answer if asked which side is the correct way, it would create a double negative and his answer would end up being the correct way.

The person telling the truth will tell the correct way when asked for it, and will truthfully reveal the correct way as his answer to such a question.

Therefore both people will give the correct answer.

stefan| Guest March 18th, 2007 - 2:54 amI’d say Joe is the closest, but it can be refined like this: ask either of them to point the direction that the OTHER would say the right way to your town was. The truth-teller would point the wrong way, as would the liar. Then you take the other road. (or you could ask about the wrong way to the town, the go that way).

scott| Guest March 22nd, 2007 - 1:44 pmthe one question to ask-regardless of whom you ask would be “which way would the other person tell me to take?” then take the opposite way, because if you ask the liar he will tell you the opposite of what the truthful person would tell you-hence the wrong way, and if it is the truthful person, he would tell you the direction the liar would answer, which would be the wrong way. so both would give you the wrong direction, no matter which you ask, so you would take the opposite way. you would only have to ask one question to either person.

Josh| Guest April 1st, 2007 - 7:47 amBeing able to only ask one of the two guys one question, asking “which road would the other guy say to take” and going down the opposite of his answer would get you where you needed to go correctly everythime.

james| Guest April 27th, 2007 - 4:58 pmhttp://www.xkcd.com/c246.html

alison| Guest August 4th, 2007 - 7:49 amWhy dont you ask them a question you already know the answer too, like, “am I wearing a pink shirt today”? obviously the guy who tells the truth will answer correctly and the guy who lies wont!! Then you go the way the truth guy tells you, simple!!

Dan| Guest August 21st, 2007 - 1:12 pmI think Joe’s answer is correct. If I am reading his answer correctly, when both answer “NO” to “Will the other guy tell me that this is the right road to take to get to town?”, the path is the correct one.

When they both answer “YES”, the path is wrong.

Mike| Guest September 4th, 2007 - 8:10 amYou need to ask any of the guys the question:

“If I asked the other guy which road leads to the one I want to get to, what would he say?”

If you ask this the liar, he would lie and say that the other guy would say the left, when it is in fact right.

If you ask the truthful guy, he would say the truth and say the other guy would say left. You know the other guy is a liar, so the answer is right.

Andi| Guest September 11th, 2007 - 7:38 amYou can ask just one of them (doesn’t matter which one) “If I go left and it is correct will the other tell me it is correct?

No matter who you are asking, if it is the correct way to go, both of them would answer this question “NO”

If it is the wrong way then either person will answer “YES”

Matt| Guest September 26th, 2007 - 5:23 pmIf your goal is to go to truth town – simply ask “Which road did you come from?” The lair will point you to Truthtown because he always lies and the truthteller will also point you to truth town because he never lies.

Also – the premise of this puzzle is that you get only ONE question.