A Simple Brain teaser

dc0d32 · Post by **dc0d32** » Sun Jul 08, 2007 2:40 am

You are rowing down a river, and see a fork. One sub-stream ends up in a normal calm river, while the other continues to a fall. On the center of the fork lies a small hut. Two identical twins live there. One always speaks the truth, the other always lies, and you dont know who is who. They know their surroundings well.

You can ask only one of them a single question.

What will you ask?

Tyler · Post by **Tyler** » Sun Jul 08, 2007 6:38 am

"If i ask the other guy will he tell me your mum is fit"

The liar will tell me No if she is because he will lie abotu the Truth sayers answer. The truth sayer will also say no because he knows the liar will say no because he lies. aAlternatively the mum is not hot and they both say Yes.

Of course the entire thing is a impossible, because the liar should infact realise his brother would realise he would say no and therefore also say no, so the liars answer should become yes which in turn wold change the truth sayers answer to yes and the liars to no... it goes on like ths for a few million years (eternity actually).

dc0d32 · Post by **dc0d32** » Sun Jul 08, 2007 7:40 am

lol

i assure you there is a solution . You can save your life.

Colonel Kernel · Post by **Colonel Kernel** » Sun Jul 08, 2007 11:01 am

Talk to one of the twins and ask, "would your sibling tell me that the left fork leads to the falls?" If the answer is "no", take the left fork. If the answer is "yes", take the right fork.

This has been posted recently in another thread, hasn't it...?

XCHG · Post by **XCHG** » Sun Jul 08, 2007 11:59 am

Colonel Kernel wrote:Talk to one of the twins and ask, "would your sibling tell me that the left fork leads to the falls?" If the answer is "no", take the left fork. If the answer is "yes", take the right fork.

This has been posted recently in another thread, hasn't it...?

That can not be the correct answer. Suppose:

1) The right sub-stream is the calm river.
2) The left sub-stream is the one that ends to a fall.
3) You are talking to the twin who says the truth.

Now if you ask him "would your sibling tell me that the left fork leads to the falls?" he would of course say No because the other twin is the liar and the route to the left does lead to the fall so the liar is going to say No and so will the one who tells you the truth. Now according to your solution you take the left route and you are dead meat

piranha · Post by **piranha** » Sun Jul 08, 2007 10:35 pm

Go up to any one twin and ask "If you were your twin, what sub-river would you tell me to take for safety?" Then go the other way. Works in all situations.

Colonel Kernel · Post by **Colonel Kernel** » Sun Jul 08, 2007 11:51 pm

XCHG wrote:Now if you ask him "would your sibling tell me that the left fork leads to the falls?" he would of course say No because the other twin is the liar and the route to the left does lead to the fall so the liar is going to say No and so will the one who tells you the truth. Now according to your solution you take the left route and you are dead meat

Doh... I got left & right backwards.

Where did all my missing brain cells go? Maybe into the gaping hole in my sleep schedule...

Tyler · Post by **Tyler** » Mon Jul 09, 2007 12:23 pm

That is all exactly what i said... except that i also proved the entire thing doesn't really work. I hate when people post the answer over and over.

Colonel Kernel · Post by **Colonel Kernel** » Mon Jul 09, 2007 12:55 pm

Tyler wrote:Of course the entire thing is a impossible, because the liar should infact realise his brother would realise he would say no and therefore also say no, so the liars answer should become yes which in turn wold change the truth sayers answer to yes and the liars to no... it goes on like ths for a few million years (eternity actually).

This is not possible unless the liar can choose to answer a different question than the one that was asked. I think one of the assumptions of the problem is that the liar is compelled to answer exactly the question you ask, even though the answer given is wrong.

Remember, the question is not, "would your sibling say that you would say that he would say that you would say... etc." There is only one level of indirection involved in the question.