Jump to content


Welcome to BrainDen.com - Brain Teasers Forum

Welcome to BrainDen.com - Brain Teasers Forum. Like most online communities you must register to post in our community, but don't worry this is a simple free process. To be a part of BrainDen Forums you may create a new account or sign in if you already have an account.
As a member you could start new topics, reply to others, subscribe to topics/forums to get automatic updates, get your own profile and make new friends.

Of course, you can also enjoy our collection of amazing optical illusions and cool math games.

If you like our site, you may support us by simply clicking Google "+1" or Facebook "Like" buttons at the top.
If you have a website, we would appreciate a little link to BrainDen.

Thanks and enjoy the Den :-)
Guest Message by DevFuse
 

Photo
- - - - -

The liar, the truth teller....and the random answerer


Best Answer Writersblock , 30 July 2007 - 01:31 AM

Martini,

Spoiler for ...
Go to the full post


  • Please log in to reply
94 replies to this topic

#51 ShawnInToronto

ShawnInToronto

    Newbie

  • Members
  • Pip
  • 8 posts

Posted 16 August 2008 - 04:05 AM

Ask person A the first question:

"Would you currently answer yes to the statement 'I sometimes tell the truth and sometimes lie' ?"

If A is the truth teller, he will truthfully say no.
If A is the liar, he would say yes to 'I sometimes tell the truth and sometimes lie', but will lie about his agreement, answering no.
If A is the random in the truth-telling mood, he would truthfully answer yes.
If A is the random in the lying mood, he would not answer yes to 'I sometimes tell the truth and sometimes lie', but will lie and say he would, answering yes.

If A answered yes, we know he is random. We then ask B a second question 'Would C say that you are a liar ?'. If B says yes, he is the truth-teller and C is the liar; if he says 'no' he is the liar and C is the truth-teller. In this case we know the identies of all three with two questions.

If A answered no, we know he is either the truth-teller or the liar. Ask him a second question 'Would B or C ever agree that you are a liar ?". If yes , A is the truth-teller. If no, A is the liar. Know we know whether A is a liar or truth-teller with one question left. Ask him the last question, 'Is B random ?'.

If A is the truth-teller, yes implies B random, C liar. No implies B liar, C random.

If A is the liar, yes implies B liar, C random. No implies C liar, B Random.

All three identies known with only 3 yes / no questions.
  • 0

#52 Prime

Prime

    Senior Member

  • Members
  • PipPipPipPip
  • 872 posts
  • Gender:Male
  • Location:Illinois, US

Posted 16 August 2008 - 05:46 PM

This is a very interesting problem. After checking the posts, I see that Writersblock had solved it a year ago. The solution that I found is the same in essence. Though, I found a different kind of relation between the three men (other than LEFT RIGHT).
Spoiler for Another relation:

Edited by Prime, 16 August 2008 - 05:47 PM.

  • 0

Past prime, actually.


#53 aly410

aly410

    Newbie

  • Members
  • Pip
  • 8 posts

Posted 17 September 2008 - 03:11 PM

i have a solution too and it seems simple:D

so:
we have 1 2 3
We ask 1:" do i have 5 figers at one hand?" ( and i do :)) )
He answers: NO => 1 = L
then i ask 1 : " is 2 random?" => he answers yes =>2= T and 3 = R
no => 2 = R ; 3 = T

He answers YES: => 1 = T or R
then i ask 2" is 1 L?" => YES => 2 = L then i ask 2" is 1 T?" => YES => 1= R and 3= T
NO => 1= t and 3= R
=> NO => 2 = R or T and 3 = L ; so i ask 3 "if 1 is T?" = > YES => 1=R and 3=T
NO => 1= T and 3 = R

Is it OK :D?
  • 0

#54 achemaromba

achemaromba

    Newbie

  • Members
  • Pip
  • 3 posts

Posted 03 November 2008 - 03:59 AM

establish which man is the truth-teller. ask man1(m1) if he always speaks truth. if he replys no ask m2 the same question. if he replys no, u know m3 is the truth-teller. ask him if the man to his left (or right) always lies. weather he replies yes or no you can deduce who the remaining 2 men are. if u find the truth teller on the first question u can find out in 2 questions
  • 0

#55 Martini

Martini

    Senior Member

  • Members
  • PipPipPipPip
  • 770 posts

Posted 03 November 2008 - 04:35 AM

establish which man is the truth-teller. ask man1(m1) if he always speaks truth. if he replys no ask m2 the same question. if he replys no, u know m3 is the truth-teller.

That won't work; two men can't answer that question with a "no".
  • 0

#56 Phatfingers

Phatfingers

    Advanced Member

  • Members
  • PipPipPip
  • 188 posts

Posted 20 January 2009 - 04:02 AM

i have a solution too and it seems simple:D

so:
we have 1 2 3
We ask 1:" do i have 5 figers at one hand?" ( and i do :)) )
He answers: NO => 1 = L
then i ask 1 : " is 2 random?" => he answers yes =>2= T and 3 = R
no => 2 = R ; 3 = T

He answers YES: => 1 = T or R
then i ask 2" is 1 L?" => YES => 2 = L then i ask 2" is 1 T?" => YES => 1= R and 3= T
NO => 1= t and 3= R
=> NO => 2 = R or T and 3 = L ; so i ask 3 "if 1 is T?" = > YES => 1=R and 3=T
NO => 1= T and 3 = R

Is it OK :D?


You have a problem if 1 is Random. He might answer "NO".
  • 0

#57 Phatfingers

Phatfingers

    Advanced Member

  • Members
  • PipPipPip
  • 188 posts

Posted 20 January 2009 - 07:38 AM

There is a truth teller (always tells the truth), a liar (always lies), and one that sometimes answers truthfully and sometimes lies. Each man knows who is who. You may ask three yes or no questions to determine who is who. Each time you ask a question, it must only be directed to one of the men (of your choice). You may ask the same question more than once, but of course it will count towards your total. What are your questions and to whom will you ask them?


Spoiler for Yet another solution...

  • 0

#58 Prime

Prime

    Senior Member

  • Members
  • PipPipPipPip
  • 872 posts
  • Gender:Male
  • Location:Illinois, US

Posted 20 January 2009 - 08:04 AM

Spoiler for Yet another solution...

Just looking at your first case, NNN could mean A=Truth-teller, B=Random, C=Liar; or A=Random, B=Truth-teller, C=Liar. So those three questions do not resolve who is who. Correspondingly, your arrangement for the YNY implies that the Truth-teller lied to the first question. As well the YNN arrangement implies that the Liar answered the first question truthfully...

Edited by Prime, 20 January 2009 - 08:11 AM.

  • 0

Past prime, actually.


#59 Phatfingers

Phatfingers

    Advanced Member

  • Members
  • PipPipPip
  • 188 posts

Posted 20 January 2009 - 03:59 PM

Just looking at your first case, NNN could mean A=Truth-teller, B=Random, C=Liar; or A=Random, B=Truth-teller, C=Liar. So those three questions do not resolve who is who.

If A is Random, then his "No" answer to Q1 is a lie.
If C is Liar, then his affirmation of Q2 would be "Yes".
B therefore must answer yes, making "NNN" an impossibility for combination RTL.

Correspondingly, your arrangement for the YNY implies that the Truth-teller lied to the first question. As well the YNN arrangement implies that the Liar answered the first question truthfully...

Got me there. I originally solved it where A and C were asked the questions of B and I tried to swap them in my head as I was transcribing. Let's see if it was a transcription error or a logic error:

YNY should imply:
A is determined non-random, therefore A is asked second question, and proven truthful.
Truthful A claimed C is Random, so C is Random and B is Liar.
YNY should have equated to A=Truthteller, B=Liar, C=Random

YNN should imply:
A is determined non-random, therefore A is asked second question, and proven a liar.
Liar A claimed C is Random, so C is Truthteller and B is Random.
YNN should have equated to A=Liar, B=Random, C=Truthteller

Yup. I shouldn't try to sort and copy at the same time... I still assert (randomly?) that the logic is sound. The questions are sufficient to determine who's who.
  • 0

#60 IDoNotExist

IDoNotExist

    Senior Member

  • Members
  • PipPipPipPip
  • 1304 posts

Posted 20 January 2009 - 06:03 PM

There is a truth teller (always tells the truth), a liar (always lies), and one that sometimes answers truthfully and sometimes lies. Each man knows who is who. You may ask three yes or no questions to determine who is who. Each time you ask a question, it must only be directed to one of the men (of your choice). You may ask the same question more than once, but of course it will count towards your total. What are your questions and to whom will you ask them?


Spoiler for Answer<br />


If (2) answers 'no' than (2) is the liar. Point to one of the men you know to be either a truth teller or random, at this point in the example either (1) or (3), and ask the liar, (2) if the man you are pointing to is a truth teller. If the liar says no, the man you are pointing to is the type of man you asked about. If the liar says yes, than he is the other man.

Hope that made sense.
  • 0




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users