A logic puzzle

I can't remember where I first heard this, but I remember having fun with it.

You're in a room that has 2 computers inside. There are also 2 doors, 1 of them leads to an eternity of your worst fears, the other to an eternity of bliss and you don't know which is which. You are allowed to ask 1 computer 1 question, knowing that 1 computer always lies and the other always tells the truth, but not which is which.

What's the question? I guess there could be more than 1 right question, but I've only found 1 that works.

Good luck BTSers!!!

You ask computer 1 to tell you what computer 2 think is the correct door.

If computer 1 tell you a different answer than computer 2, you know #1 is the liar and #2 gave the correct answer.

If #1 repeat what #2 said, you know #1 is telling the truth but #2 tried to mislead you, you take the other door.

I have thought of alot of different questions to ask but i cant figure it out. Can you U2U the answer to me

I think you worded the question incorrectly. It should read, "You can ask each computer one question."

ufia's answer was correct. You ask one computer which is the correct door. Then you ask the second computer what the first computer just said. If the second computer repeats the first computer's answer truthfully, then you know the first computer is the liar and you choose the door that it didn't recommend. If the second computer says something different to the first, you know the second computer is the liar and you should indeed choose the door the first computer recommended.


[edit on 2006-6-18 by wecomeinpeace]

Does that work out?

Say door 1 = heaven and door 2 = hell (but you won't know that of course).

If computer 1 is the honest one and you ask it 'what does computer 2 think is the heaven door' then computer 1 will tell you that computer 2 thinks door 2 is the heaven door, and when you ask computer 2 they will indeed tell you that door 2 is the heaven door.

If computer 1 is the liar and you ask it the same question, it's going to tell you that computer 2 will give you the answer 'door 2'. You then ask computer 2 what the heaven door is and it says 'door 1'.


I understand logic gates fine but something about that seems wrong. Maybe the wording just screws with my thought process.

If computer 1 is the honest one and you ask it 'what does computer 2 think is the heaven door' then computer 1 will tell you that computer 2 thinks door 2 is the heaven door, and when you ask computer 2 they will indeed tell you that door 2 is the heaven door.

So computer 1 didn't lie. So what does this tell you? It tells you that compter 1 is the honest computer, and hence computer 2 is the liar. So now you know that the door computer 2 (liar) tells you to choose is the WRONG door. You choose the other one.

If computer 1 is the liar and you ask it the same question, it's going to tell you that computer 2 will give you the answer 'door 2'. You then ask computer 2 what the heaven door is and it says 'door 1'.

So computer 1 lied. You know it is the liar. Therefore, you know that computer 2 is the honest one. So you should take computer 2's advice and choose whichever door it tells you.




[edit on 2006-6-18 by wecomeinpeace]

One Question Could Do It

Ask either computer what the other computer would tell you is the door to eternal bliss.

Whatever the answer is, choose the other door.

1. If you ask the honest computer, it will tell you truthfully the lie the other computer would tell you.

2. If you ask the dishonest computer, it would tell you falsely that the other computer would tell you the wrong door.

Thus in either case, the answer will be false.

So with one question, you're set.

Nice.

Something about it doesn't join-up in my head. It seems to be a case of you have to know in advance that (in the two-questions example) an answer where both computers say the same thing means pick door whatever (determined by the answer given at the time), whereas a reply where there are two different answers means that you will use those replies to work out what door to pick.

So it's like you must know in advance the formula - two same answers means pick the door not in those two same answers, two different answers means - well I can't figure out how you would determine what one in that case was the honest one and what one wasn't.

Like when you say 'so computer 1 didn't lie' and 'so computer 1 lied' - but you won't know that unless you actually go through the door, then it's too late if it had lied.


The single question solution would be easy to remember - as a formula - because you just pick the opposite of whatever either computer answers.

I think my brain hates these kinds of things because in reality no-one is allowed to put anyone in that sort of position anyway and in such a scenario for real, the correct choice would be to refuse to go along with the headgames.

Majic nailed it. Those other answers hurt my brain . . . :bash:

Originally posted by ed 209
Like when you say 'so computer 1 didn't lie' and 'so computer 1 lied' - but you won't know that unless you actually go through the door, then it's too late if it had lied.

The lie you are detecting is in relation to what the other computer just said, not in relation to the doors.

[edit on 2006-6-19 by wecomeinpeace]

Well done Majic.