# that 2 guards 2 doors puzzle

Found 20th Mar 2011
i cant work it out,even if you ask what the solutionn says,not normally thick but it stumped me.
I have asked this before, think its called knight and knave I am confused over it too but understand it in part.

Basically you have to have a trick question where the answer is the same be it a lie or truth, as if you ask the liar a question, they have to lie so if the question is asking about a lie, then the lie is the truth.

John and Bill are standing at a fork in the road. You know that one of them is a knight and the other a knave, but you don't know which. You also know that one road leads to Death, and the other leads to Freedom. By asking one yes/no question, can you determine the road to Freedom?

There are several ways to find out which way leads to freedom. One alternative is asking the following question: "Will the other man tell me that your path leads to freedom?"

If the man says "No", then the path does lead to freedom, if he says "Yes", then it does not. The following logic is used to solve the problem.

If the question is asked of the knight and the knight's path leads to freedom, he will say "No", truthfully answering that the knave would lie and say "No". If the knight's path does not lead to freedom he will say "Yes", since the knave would say that the path leads to freedom.

If the question is asked of the knave and the knave's path leads to freedom he will say "no" since the knight would say "yes" it does lead to freedom. If the Knave's path does not lead to freedom he would say Yes since the Knight would tell you "No" it doesn't lead to freedom.

The reasoning behind this is that, whichever guardian the questioner asks, one would not know whether the guardian was telling the truth or not. Therefore one must create a situation where they receive both the truth and a lie applied one to the other. Therefore if they ask the Knight, they will receive the truth about a lie; if they ask the Knave then they will receive a lie about the truth.

Edited by: "ryouga" 20th Mar 2011
Imagine door 1 is the unsafe choice.

You ask guard one "If I ask guard 2 which door is safe what would he say?"

If guard 1 is the liar he will lie that the other guard would tell you door 1. If he is the truthful one he will tell you that the other guard will tell you door 1. Either way you choose the door they don`t mention.
Why don't you just ask the guards what door they came in & both will give same door.

Why don't you just ask the guards what door they came in & both will give … Why don't you just ask the guards what door they came in & both will give same door.

Genius
Why don't you just ask the guards what door they came in & both will give same door.
yes but if both say door 1 which is the liar

If I ask guard 2 which door is safe what would he say
as he has to lie he would say door one,if i then ask guard 2 the same question (he has to tell the truth and say door one

"Will the other man tell me that your path leads to freedom?"

the liar would say yes therefore the true guy also has to say yes.
well ???
Bump................
I'm confused. One bit of the labyrinth film I never got and still don't!!

so basically the idea is that you ask one of them will the other person say their path leads to freedom.

and you do the opposite. this is because a truth and a lie equals a lie.

say the truth teller is in front of freedom and the liar is in front of death.

ask either one will the other say their path leads to freedom?

if you asked the liar he knows the truth teller is in front of freedom and so the truth teller would say "yes" so the liar says "No".

if you asked the truth teller as he knows the liar is in front of death the liar will say "Yes" and so the truth teller will say "yes".

if the liar was in front of freedom the same situation applies. just this time the truth teller would say no and the liar would say yes.

hope this helps
This was asked last week only with angels. The answer is easy to find. Just google what you know. That's what everyone who claims to have worked it out has done.
simples, walk up to one them hold up two fingers and ask them how many fingers are holding up.
what is your date of birth
how many fingers am i holding up?

that is genius you only need to ask one person to get the right answer.

Nope, because they can only answer yes or no.

Also, even if you establish which one is the liar you don't know which one is infront of freedom and which one is infront of death.

false prophet

how many fingers am i holding up?that is genius you only need to ask one … how many fingers am i holding up?that is genius you only need to ask one person to get the right answer.

boabbyrab

simples, walk up to one them hold up two fingers and ask them how many … simples, walk up to one them hold up two fingers and ask them how many fingers are holding up.what colour is your hairwhat is your date of birthwhat is your name

You never put much thought in to this did you :-)
WMN - did my explanation at post 12 help?
bykergrove

WMN - did my explanation at post 12 help?

No,iz it because im'z thick :-/

No,iz it because im'z thick :-/

dude I wrote post 12 for you... you kept bumping the thread so i wrote that explanation.
darn didnt realise it was yes and no only.

ok lets break this down in simple terms

evil guards hell and can only lie thats the left door

good can only tell the truth thats the right door

lets make left bad and right good

question

left does right guard the door to heaven ,left answers no

right does left guard the door to heaven right answers no

from here how do we say which side is lying?????????????
false prophet

darn didnt realise it was yes and no only.ok lets break this down in … darn didnt realise it was yes and no only.ok lets break this down in simple termsevil guards hell and can only lie thats the left doorgood can only tell the truth thats the right doorlets make left bad and right goodquestionleft does right guard the door to heaven ,left answers noright does left guard the door to heaven right answers nofrom here how do we say which side is lying?????????????

you ask the guard what the other guard is going to say.

truth + lie = lie so you go with the opposite of what the guard you asked has said.

THE END.
ok prove it in yes or no answers

im the devil and guarding hell and can only lie

false prophet

ok prove it in yes or no answersim the devil and guarding hell and can … ok prove it in yes or no answersim the devil and guarding hell and can only lieask me a question

is the other guard guarding hell?
yes
bykergrove

is the other guard guarding hell?

the other guard (angel or whatever) is going to say "No" because they are guarding heaven

since you're the devil you're going to lie about what they say so you say "Yes" that the other guard (the angel) is guarding hell.

since i know that there is a lie involved no matter which guard I ask... I go with the opposite to what the guard i ask tells me.

so I know that the other guard is guarding heaven.

THE END?
get out of that
false prophet

yes

So I go past the other guard as i know they are guarding heaven.
no if you ask the good guy is the other bloke guarding hell he says yes because he cant lie
if you ask the bad guy he also says yes because he has to lie
false prophet

get out of that

do you understand how the solution works now?

or do you want to try it again but this time you be the angel?
get out of that
you dont get it ,thats ok some people believe what they read thats ok
false prophet

no if you ask the good guy is the other bloke guarding hell he says yes … no if you ask the good guy is the other bloke guarding hell he says yes because he cant lie

ok lets say you're the angel but we'll keep everything else the same. (devil is guarding hell)

so I ask you "is the other guard going to say they are guarding hell"

the angel knows that the devil is going to say "no" so the angel tells the truth and says "no" that the devil is going to say no they are not guarding hell

but because i know one of them is lying i choose the opposite
bykergrove

is the other guard guarding hell?

I made a mistake here... the solution is to ask the guard you are speaking to that the OTHER guard is going to say.

post 26 and 34 from there is what happens:
bykergrove

the other guard (angel or whatever) is going to say "No" because they are … the other guard (angel or whatever) is going to say "No" because they are guarding heavensince you're the devil you're going to lie about what they say so you say "Yes" that the other guard (the angel) is guarding hell.since i know that there is a lie involved no matter which guard I ask... I go with the opposite to what the guard i ask tells me.so I know that the other guard is guarding heaven.THE END?

bykergrove

ok lets say you're the angel but we'll keep everything else the same. … ok lets say you're the angel but we'll keep everything else the same. (devil is guarding hell)so I ask you "is the other guard going to say they are guarding hell"the angel knows that the devil is going to say "no" so the angel tells the truth and says "no" that the devil is going to say no they are not guarding hellbut because i know one of them is lying i choose the opposite

oxymoron
false prophet

oxymoron

it's not an oxymoron
false prophet

you dont get it ,thats ok some people believe what they read thats ok

ok

1=god truth heaven

1:if i ask 2 if he is guarding the door to heaven what would he say,he knows 2 can only lie therefore he has to say "yes" because thats the only answer 2 can give

2:if i ask 1 are they are guarding the door to heaven he can only say no as he has to answer untruthfully

it would work like that good would give a wrong answer because he has to say what bad says making hhis answer correct in hindsight.

still think its a bit dodgy tho
although point 1 is incorrect in that number 2 isnt guarding heaven your asking what he would say,1 knows 2 isnt guarding heaven but he knows 2 has to lie and only answer the question given