2 guards, 2 doors - Earth Empires Forum

Fooglmog

Member
1149

Nov 10th 2012, 2:04:24

I'm making it a hobby to take well known riddles, and adding twists so the classic answer doesn't work.

2 guards stand between two doors. One door leads to paradise, the other to death and suffering. One guard always tells the truth, and the other always lies. You don't know which is which. You can ask one of them, one question. What do you ask to determine which door leads to paradise?

The twist: The guards do not know anything about each other, and so can not tell you how the other would answer.

I just came up with this a few minutes ago while I was reading today's XKCD comic and it is (as far as I know) original. See if you can work it out.

-Fooglmog
Guy with no clue.

Edited By: Fooglmog on Nov 10th 2012, 2:28:15

bstrong86

Member
2482

Nov 10th 2012, 2:20:18

push the liar through a door and ask the turth teller what happened...?

The Death Knights

XI

Pride

Member
1590

Nov 10th 2012, 2:29:14

Ask the guard if he would walk through his door?

http://www.boxcarhosting.com/...application.php?clanID=MD

Fooglmog

Member
1149

Nov 10th 2012, 2:29:50

Sorry, part of the original is that you don't know which is which. I forgot to include that.

-Fooglmog
Guy with no clue.

Fooglmog

Member
1149

Nov 10th 2012, 2:31:38

Originally posted by Pride:

Ask the guard if he would walk through his door?

They'd either both say yes, or both say no... so this doesn't really help.

-Fooglmog
Guy with no clue.

Rockman

Member
3388

Nov 10th 2012, 2:52:40

Is it an African Swallow, or a European Swallow?

LittleItaly

Game Moderator
Alliance, FFA, & Cooperation
2197

Nov 10th 2012, 2:58:03

just ask the guard if they can open their door without pushing you in so you can peek inside.

or

tell them you are a door repair man, and god sent you to repair the door. have them take the door off for you to repair the hinges, which will allow you to peek inside to potentially run in or chose to run away.

LittleItaly
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Helmet

Member
1341

Nov 10th 2012, 2:58:23

You ask one of them if OJ did it. If they say yes, you move forward, if he says no you go to the other door.

Alin is a cheater.

http://i1090.photobucket.com/...ainoftse/alincheating.png

Anonymous

Member
384

Nov 10th 2012, 3:18:14

Originally posted by Helmet:

You ask one of them if OJ did it. If they say yes, you move forward, if he says no you go to the other door.

The liar isn't necessarily guarding the door to hell. That's why you need to get out of them which door is to heaven.

Devestation

Member
812

Nov 10th 2012, 3:23:41

Kill one. He'll therefore be sent through one of the doors. After that, ask the other guard where the dead guy went.

Although I suppose they'd both just say hell, actually.

Devestation

Member
812

Nov 10th 2012, 3:25:09

Oh, of course. Ask one guard which door they'd be sent through if you killed them. They'll both point at Paradise.

Anonymous

Member
384

Nov 10th 2012, 3:28:59

Originally posted by Devestation:

Oh, of course. Ask one guard which door they'd be sent through if you killed them. They'll both point at Paradise.

Hmm... I think that's the best we will get. Makes perfect sense if the original wording is used about one being an angel and the other a devil, and the doors leading to heaven or hell.

Fuji

Member
301

Nov 10th 2012, 3:41:35

If I was wearing white shoes, I would ask them both "are my shoes white?". The one who said yes I would go into his door.

Edit: This is assuming the one who tells the truth is guarding the door to paradise. Upon further reading you don't say that. Uh oh!

Edit 2: Actually, once knowing what guard is telling the truth I can simply ask him what door holds paradise! I win!?

Edit 3: I lack basic reading comprehension.

Edited By: Fuji on Nov 10th 2012, 7:16:21
See Original Post

Devestation

Member
812

Nov 10th 2012, 4:57:54

You can send my prize to Big Ben, btw.

Anonymous

Member
384

Nov 10th 2012, 6:59:51

Originally posted by Fuji:

Edit 2: Actually, once knowing what guard is telling the truth I can simply ask him what door holds paradise! I win!?

You only get one question to each guard.

Fuji

Member
301

Nov 10th 2012, 7:08:55

Damnit!

Pain

Member
4849

Nov 10th 2012, 7:27:34

Originally posted by Anonymous:

Originally posted by Fuji:

Edit 2: Actually, once knowing what guard is telling the truth I can simply ask him what door holds paradise! I win!?

You only get one question to each guard.

you only get 1 question for 1 guard.

Your mother is a nice woman

Xinhuan

Member
3728

Nov 10th 2012, 7:35:22

Use "double negative" to make the liar lie about lying so that the answer is the truth. Ask the following question:

Would you answer 'yes' to the question 'Does this door go to heaven?'

-----------------------------
Scenario 1: Question was posed to Truth guarding Paradise
'Does this door go to heaven?' - Yes
'Would you answer yes?' - Yes

Scenario 2: Question was posed to Truth guarding Hell
'Does this door go to heaven?' - No
'Would you answer yes?' - No

Scenario 3: Question was posed to Liar guarding Paradise
'Does this door go to heaven?' - No (lies)
'Would you answer yes?' - Yes (lies about answering yes)

Scenario 4: Question was posed to Liar guarding Hell
'Does this door go to heaven?' - Yes (lies)
'Would you answer yes?' - No (lies about answering yes)

Conclusion: If the guard answered Yes, enter the door behind him. Else enter the other door.

Pain

Member
4849

Nov 10th 2012, 7:39:51

thats 2 questions

Your mother is a nice woman

Xinhuan

Member
3728

Nov 10th 2012, 7:40:58

It's only 1 question Pain.

Would you answer 'yes' to the question 'Does this door go to heaven?'

The guard has to answer the outer question, not the inner one.

Pain

Member
4849

Nov 10th 2012, 7:50:17

if im the liar standing next to the door to paradise and you ask me that question im going to say no. how do you determine im not actually the truth teller telling the truth about the door behind me not going to heaven?

Your mother is a nice woman

Xinhuan

Member
3728

Nov 10th 2012, 7:53:12

The thing is, even without the Twist, it is still the correct answer (out of 2 possible correct answers).

The classic answer would be to ask the guard about how the other guard would answer, but the the second correct answer which is simply to ask how he himself would answer.

Classic answer:

"If I were to ask the other guard which door leads to heaven, which door would he answer?"

The classic answer assumes the 2 guards know about each other.

Alternate answer:

"If I were to ask you which door lead to heaven, which door would you answer?"

The alternate answer makes no such assumption.

I merely rephrased the alternate answer.

Xinhuan

Member
3728

Nov 10th 2012, 7:55:01

Originally posted by Pain:

Pain, the goal isn't to determine who is the truth guy, and who is the liar. The goal is to determine which door leads to heaven.

The way my question is constructed, you will never know which guard was which. But you will know which goes to heaven.

Pain

Member
4849

Nov 10th 2012, 7:57:18

why do each of your scenarios have 2 anwsers for 1 question though? you cannot have 2 answers to 1 question otherwise its not 1 question.

maybe im confused

Your mother is a nice woman

Xinhuan

Member
3728

Nov 10th 2012, 8:00:57

Ya, you are. The first answer is the answer to the inner question which the guard doesn't have to answer. The second answer is the actual answer they would tell you.

Note that the rules of the riddle is that the liar MUST lie. It is not his job to mislead, it is his job to provide incorrect answers. Thus asking the liar to lie about lying will give you correct information.

If you are still confused, try "If I were to ask you which door lead to heaven, which door would you answer?". Enter the door he points at, doesn't matter if he was liar or not.

Pain

Member
4849

Nov 10th 2012, 8:01:08

also your classic and alternate answer statements only work if you know which one is the liar and which one is the truth teller, which you do not.

Your mother is a nice woman

Frodo

Member
405

Nov 10th 2012, 8:03:16

Originally posted by Xinhuan:

Alternate answer:

"If I were to ask you which door lead to heaven, which door would you answer?"

The alternate answer makes no such assumption.

I merely rephrased the alternate answer.

Ok so heres how i think it would go
1). Liar guarding Paradise: He would point at the other door.
2). Liar guarding Hell: He would point at his door
3). Truthful guarding Paradise: He would point at his door
4). Truthful guarding Hell: He would point at other door.

So maybe I am missing something simple, along with Pain, but I see no way here how you can determine, by just asking one guard one question, which door is which.

Frodo

Member
405

Nov 10th 2012, 8:06:00

Originally posted by Pain:

also your classic and alternate answer statements only work if you know which one is the liar and which one is the truth teller, which you do not.

Yeah that is the problem i see too. Because you could ask the truthful guard and he would point at the correct door... while the liar would point at the wrong door.... But you dont know which one is which. So in my opinion you need to ask a question which 1st determines whether you are talking to the liar, and secondly a question that gives you an answer one way or the other.

Xinhuan

Member
3728

Nov 10th 2012, 8:09:54

You're missing it Frodo.

1). Liar guarding Paradise: He would point at the his own door.
2). Liar guarding Hell: He would point at the other door.

Because the Liar has to double lie. Consider this in scenario (1), if you were to simply ask

(a) "which door lead to heaven?"

He would point at the other door for this simple question. But now you wrap that question like this:

(b) "How would you answer question (a) above?"

He has to lie and point to his own door. Get it? You don't actually ask question (a), and he never answers (a). You ask question (b) which asks about question (a), and he answers (b).

Again, the guard must LIE. The guard is NOT trying to mislead you, if his job is to mislead you, then yes, he would have answered the way you posted. Again, you don't have to find out who is the liar, and the solution doesn't require you to.

Frodo

Member
405

Nov 10th 2012, 8:13:30

I think I get it haha, your scenario thing is laid out a little confusing because it appears that the guard gives two answers. Only flaw i see in this is if the guard doesnt think through the question and answers based on the inner question then you will go through the wrong door... but its a hypothetical question so we can assume the guards are really smart haha.

Frodo

Member
405

Nov 10th 2012, 8:15:51

I am quite impressed with your logic there, very well done. I dont think I ever would have thought of that haha.

Pain

Member
4849

Nov 10th 2012, 8:20:28

Would you answer 'yes' to the question 'Does this door go to heaven?'

youre pointing at a door (paradise for example). you ask him if that door goes to paradise he would say no (normally). if you ask him if he would say yes to that door doing to heaven hes going to say yes because he would really say no if you asked him that question directly because he has to lie about what he would really say. i get that part. i dont get how you determine that he would have said no to the question of if that door went to paradise. youre assuming at that point.

Your mother is a nice woman

Pain

Member
4849

Nov 10th 2012, 8:22:54

nevermind i got it. now we just need to see if using a double negative as a question is acceptable :P

Your mother is a nice woman

Iamminghui

Member
176

Nov 10th 2012, 10:07:55

Hmmm.. Since one is Lying and the other is telling the truth, you ask the first one 'is 1+1=3'... If he answers 'yes' then u know he is the lying one, hence u ask the other one 'is this the stairway to heaven' and he will tell u the truth..

On the other hand, if the first one answers 'no' means he is the one telling the truth, then the other guard is the liar, hence when u ask the second question, u just follow the opposite answer that he gave..

I think this settles it..

Iamminghui

Member
176

Nov 10th 2012, 10:09:05

Oh.. Not heaven.. To paradise I mean..

galleri

Game Moderator
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Nov 10th 2012, 11:25:39

Where is Asgaard in this?

https://gyazo.com/...b3bb28dddf908cdbcfd162513

Kahuna: Ya you just wrote the fkn equation, not helping me at all. Lol n I hated algebra.

Rockman

Member
3388

Nov 10th 2012, 13:02:00

The problem with Xinhuan's solution is that his question will confuse the guard. If the guards were computers, Xinhuan's technique would work. If they were people, Xinhuan would have to ask the question about 15 times just to get an answer.

Atryn

Member
2149

Nov 10th 2012, 17:30:20

This is in the movie Labryinth, only the guards do "know about each other"... in fact, to state that the guards do not know about each other is itself a logical fallacy.

If the guard knows about their own door, and knows about themselves, they MUST also know about the other door and about the other guard since both are binary.

Xinhuan

Member
3728

Nov 10th 2012, 21:08:22

Well Atryn, Fooglmog could have just reworded "the twist" that you may not ask a guard on how the other guard will answer. :P

Jaaamm

Member
65

Nov 10th 2012, 22:06:44

bovered?

Frodo

Member
405

Nov 10th 2012, 22:30:39

Originally posted by Iamminghui:

No it doesn't you get to ask 1 question to 1 guard... not 1 question for each guard.

Anonymous

Member
384

Nov 10th 2012, 23:31:12

... You get to ask both guards one question.

Pain

Member
4849

Nov 11th 2012, 0:49:11

Originally posted by Fooglmog:

You can ask ONE of them, ONE question. What do you ask to determine which door leads to paradise?

i capitalized the important part. you get one single question for either one, not 1 question for each.

Edited By: Pain on Nov 11th 2012, 0:55:09
See Original Post

Your mother is a nice woman

Iamminghui

Member
176

Nov 11th 2012, 0:51:31

Oh.. Ok..

Anonymous

Member
384

Nov 11th 2012, 2:06:56

Originally posted by Pain:

Originally posted by Fooglmog:

You can ask ONE of them, ONE question. What do you ask to determine which door leads to paradise?

i capitalized the important part. you get one single question for either one, not 1 question for each.

Yeah this is worded better.

You get one question and can ask both guards that one question.

blid

Member

EE Patron
9319

Nov 11th 2012, 2:15:42

Devestation possibly and Xinhuan definitely seem to have solved it.

Originally posted by Mr. Titanium:

Watch your mouth boy, I have never been accused of cheating on any server nor deleted before you just did right there.

Fooglmog

Member
1149

Nov 11th 2012, 2:39:43

Xinhuan definitely came up with the solution I have in mind.

-Fooglmog
Guy with no clue.

martian

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Nov 11th 2012, 14:02:00

flarf

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RUN IT IS A KILLER BUNNY!!!

Garry Owen

Member
853

Nov 12th 2012, 1:36:38

You ask either guard "If I ask the OTHER guard which door leads to heaven, which door will he point to?" and then go in the other door.

The truthful guard will have to point to hell, since he is answering what the OTHER guard would lie about.
The lying guard will point to hell, since he is lying about which door the honest guard would point to.

Fooglmog

Member
1149

Nov 12th 2012, 1:57:50

Originally posted by Fooglmog:

The twist: The guards do not know anything about each other, and so can not tell you how the other would answer.

?

The point was that that answer doesn't work.

-Fooglmog
Guy with no clue.