You are in a romm with two doors. In the room are two people. One person will only ever tell you a lie. The other will only ever tell you the truth.

One door is the road to eternal happiness the other the road to eternal damnation.

You can ask one question only to both people (the same question) to establish which door leads to eternal happiness.

What would be your question?

Originally posted by bonny
You are in a romm with two doors. In the room are two people. One person will only ever tell you a lie. The other will only ever tell you the truth.

One door is the road to eternal happiness the other the road to eternal damnation.

You can ask one question only to both people (the same question) to establish which door leads to eternal happiness.

What would be your question?
I would point to a door and ask:
"If I were to ask you if that door leads to the road to eternal happiness, would you say yes?"

If the person always tells the truth, they will respond yes if the door is the road to paradise, and no if it is not.

If the person always lies, and the door does lead to paradise, they would also respond yes, because they would have to lie about the door leading to paradise, but would also have to lie about their response.

If that makes sense...

No, Don't make it too difficult. One person will always lie and the other tell the truth to your one question. You do not know who the liar is or the truthful one.

does one of the two people, actually include yourself?

Alternatively, you ask:

If I were to ask the other if that door led to eternal happiness what would his answer be?

If it was the door to eternal happines they would both answer no and conversely if it wasn't they would both answer yes.

no, you are in the room with a liar and a truth teller.

Originally posted by bonny
No, Don't make it too difficult. One person will always lie and the other tell the truth to your one question. You do not know who the liar is or the truthful one.
It doesn't matter. If you asked that question, you would always get a positive response if you were pointing at the correct door, and a negative answer if you weren't.

Originally posted by max
Alternatively, you ask:

If I were to ask the other if that door led to eternal happiness what would his answer be?

If it was the door to eternal happines they would both answer no and conversely if it wasn't they would both answer yes.

BINGO!!

Originally posted by bonny
BINGO!!

Or you could just ask them which one the other would say is the correct door. Then go through the opposite door.

I thought I recognised this riddle. I posted it here:

http://www.sheffieldforum.co.uk/showthread.php?postid=11735#post11735

some time ago. Oops.:blush:

My answer still works though.

And your answer relies on the two people in the room knowing that one is a liar, mine doesn't matter.

Originally posted by Sidla
I would point to a door and ask:
"If the person always lies, and the door does lead to paradise, they would also respond yes,


This part does not work as if you were pointing to the correct door he would say no. You are assuming that you know he is a liar, and you don't.

It's a case of a double negative = a positive and vice versa.

It does work.

Assuming the door leads to paradise. If you asked the liar if the door leads to paradise he would reply no. You ask him if his reply would be yes if you asked him if the door leads to paradise and he would have to reply yes, because in reality his reply would be no, but he'd have to lie about it.

BUt you can only ask one question!!!!!!

Originally posted by bonny
BUt you can only ask one question!!!!!!

Sidla, take a deep breath and ... leave it, it's not worth it.

You're all wrong !! ... they would both be liars ...

Originally posted by Sidla
It does work.

Assuming the door leads to paradise. If you asked the liar if the door leads to paradise he would reply no. You ask him if his reply would be yes if you asked him if the door leads to paradise and he would have to reply yes, because in reality his reply would be no, but he'd have to lie about it.

I follow you Sid.

You ask the question "if I asked you, does that door lead to paradise, would you answer me 'yes'?" ... to either the liar or the truth teller.

If you had asked the truth person ... then it's obvious it's the gateway.

If it was the liar you asked ... then the following logic sequence occurs ...

1) If you had asked the liar (you don't actually ask him this) "does that door lead to paradise" ... he would reply "no".

2) You are asking him if he would reply "yes" (which he would not) ... so he would say "no" (if he was honest ... but he isn't the rat!) ... so he will lie again (cos that's just the kinda fella he is) ... and answer "yes".

So if the answer is "yes" ... then you know it's the correct door ...

However ... what if it was the wrong door ?

The truth person would say "no" ... would the liar also say "no" ?

whatever question is asked there is still only going to be a 50% chance that you select the right door.
my question would be what is 2x2 if the answer wasent 4 then id know who the liar was, not that it would be of any use at this point :rolleyes:

Originally posted by Jamie
I follow you Sid.

You ask the question "if I asked you, does that door lead to paradise, would you answer me 'yes'?" ... to either the liar or the truth teller.

If you had asked the truth person ... then it's obvious it's the gateway.

If it was the liar you asked ... then the following logic sequence occurs ...

1) If you had asked the liar (you don't actually ask him this) "does that door lead to paradise" ... he would reply "no".

2) You are asking him if he would reply "yes" (which he would not) ... so he would say "no" (if he was honest ... but he isn't the rat!) ... so he will lie again (cos that's just the kinda fella he is) ... and answer "yes".

So if the answer is "yes" ... then you know it's the correct door ...

However ... what if it was the wrong door ?

The truth person would say "no" ... would the liar also say "no" ?
Exactly Jim!! So my way does work, and it IS only 1 question!!

So my way does work, and it IS only 1 question!!

it dosent work. you still dont know who your talking to.
the liar or the truth teller?

If you had asked the truth person ... then it's obvious it's the gateway.

why would it be obvious? u could be asking the liar.
in which case the answer would be reversed and then you after ask yourself am i speaking to the truthfull person or the liar, this brings you straight back to where you started.

50% chance of picking the correct door.

Nu Skills ... I don't think you grasped what I was getting at.

Well ... what sid was getting at ... I was just agreeing that his way works.

As you rightly pointed out ... you won't know if it's the liar or not you're talking too ... but you don't need too ... cos you will know it the door is the right one.

hmm i see :confused:

Originally posted by Nu_Skillz
it dosent work. you still dont know who your talking to.
the liar or the truth teller?
It doesn't matter!

Originally posted by Sidla

"If I were to ask you if that door leads to the road to eternal happiness, would you say yes?"



After reading and re reading this I suppose (gggrrrrrr) it could be seen as correct. I blame my lack of fairness on PMT.

Saying this I do think that we need an adjudicator in on your style of question.

As it wasn't a direct question in the first part, it was split into two. (There must be a name for this type of question)