identical twins logic problem

Ozo's got it.

>>Does John lie?
To which John only will reply "No"

If John is the liar the truthteller must say Yes
If John is truthful the liar must say Yes

[The points are ozo's - I just wanted to clarify the logic!]

M :o)

"Is John honest" works as well, with only John replying yes.

Actually, The questions above only finds out who lies and who tells the truth. We can only presume John is the one who tells the truth.

Another alternate question is: "Is John here?"

Liar: "no" / The one who tells the truth: "yes"

Oops, sorry.
What I meant to say is it is not stated in the question whether John is the liar or otherwise.

Oh yeah, and since we can only ask one of them, you can't determine who is which either from the above questions.

To find out whether John is the liar or otherwise, ask "Are you John?"

I have an eleven word question that might work:

Q: "Will you say yes if I ask if you are John?"

1. John (Truth): Yes.
2. John (Liar): Yes. (As it is true that he will answer "No" to "Are you John?")
3. Brother (Truth): No.
4. Brother (Liar): No. (As it is true that he will answer "Yes" to "Are you John?")

So, John will be the one who says yes.

ozo,

>>To find out whether John is the liar or otherwise, ask "Are you John?"

If John tells the truth, both will say "Yes".
If John is the liar, both will say "No".

QED

You could point to one twin and ask the other twin "Is he John?".

If the liar is John, and you ask him this question, he will answer Yes.
If the liar is John, and you ask the truthful one this question, he will answer Yes.

If the truthful one is John and you ask him this question, he will answer No.
If the truthful one is John and you ask the liar this question, he will answer No.

So depending on whether or not you get a yes or no answer to your question, you can figure out who John is.

To figure out if John is the liar ask "Are you John?" or "Is he John?"
To figure out which twin is Jobn ask "Does John lie?" or "Is John honest?"

After looking at my answer, I realize it doesn't really answer the whole question, it just adds more info to work with. I really don't think you can ask one three-word question and get the complete answer.

The original question only asks us how to identify John.  It doesn't ask us to indicate who is the liar.
So Ozo's question fulfils the requirement.

John will always answer 'No' and  'not-John' will always answer 'Yes' - so you can identify who is who(m?).

Pete

To quote Homer Simpson.

D'OH!!!

I guess that's what happens when I read riddles first thing on a Monday.

Ask either one  " Am I John?"

or while showing then a red piece of paper.........
ask.. "Is this red?"

Sorry... cross mine off.........

My deepest apologies,
I must have been half brain dead yesterday.

Ozo and Andyalder are correct.

Does John lie?

1. John (Truth): No.
2. John (Liar): No. (As it is true that he lies)
3. Brother (Truth): Yes.
4. Brother (Liar): Yes. (As it is true that John doesn't lie)

The same for "Is John honest?", just opposite.

:}

Isn't this nearly the riddle of the door knockers in Henson's "Labryinth" with David Bowie?

I've always liked it.


--ChenkeWind~~

It is a false logic puzzle.
Kind of similar to this one:

A death row convict was asked to say a sentence on the fateful day, to determine his method of execution.
Warden: "If you lie, you will be shot, if you tell the truth, you will be hung."
What did the convict say?

I will be shot.

Pete

Consider the sequence that begins with {1, 11, 21, 1211, 111221}. Find a rule that describes the sequence, and give the next term. Obviously, there are an unlimited number of rules that can describe the sequence. The one I am thinking of is pretty slick.

Write down a sequence of 0s and 1s. Underneath each consecutive pair write a 0 if they are the same and a 1 if not. Repeat this process until you are left with a single digit. Can you predict what the final digit will be?

0 1 0 1 0 0
1 1 1 1 0
0 0 0 1
0 0 1
0 1
1

http://mathworld.wolfram.com/LookandSaySequence.html

The answer to the identical twin question is not answerable because there will always be two different situations.

A = yes, B = no
A = no,  B = yes
-------------------------
A = yes, B = yes
A = no,  B = no

To follow the criteria of the question will always give you the possibliity of finding out who the liar and the truth teller is, but not who is john. Each yes and no question will give you the same results, and you can only ask one person. I believe the question is answerable.

ellencatherine, Please come back with a response.

ellencatherine
I find I am repeating myself here, but the question does not ask us to identify who is telling the truth or otherwise.
It asks ONLY to identify which is John.
Ozo has posted an answer to this which will work.

Pete

There is no solution to the question.

Everyone is thinking of the possibility of asking one question to everybody, but the question says that you can only ask one person. Meaning you can't figure out if the person you ask is telling the truth or not because there will always two different situations, yes or no. In one situation, he could say yes, but in the other situation, he could say no.

Ozo's answer was only possible when you get to use it on both brothers.
When you use it on both, you still can't find out he tells the truth and who doesn't.

you can find out the question of who tells the truth by asking a question that you are 100% sure of the answer.
Like,
I'm I here?
Is john here?

That kind of question will tell you the truth teller, but not who is who?

There are too many possible situations to conclude to a who is who answer.

This seems to be getting needlessly introspective.

The question (as it is so far above these comments by now) was:

"There are a set of identical twins.  One always lies and one always tells the truth.  One is named John.  Which one is John?  You may ask only one of the brothers a 3 word question.  The question must be a yes/no question.  What is the question? "

We don't need to know who tells the truth or who lies, all we need is to deduce which of the brothers is John.

Using ozo's logic ...
You can ask either of the brothers once only, "Does John lie?"
To which John only will reply "No"

If John is the liar the truthteller must say Yes
If John is truthful the liar must say Yes

Therefore if the response is "No" you are talking to John if the response is "Yes" you're not!

You only need to ask the question once.  You only need to determine the brother called John, it doesn't matter whether he is the liar or not.

What matters now is where is  ellencatherine??

M :o)

wait a minute!!

Using ozo's logic ...
You can ask either of the brothers once only, "Does John lie?"

this seems flawed to me some how. It was on the top of my head and i just lost it.
I understand how everyone is coming to that conclusion, but there seems to be some kind of hidden assumption.

Does John Lie?

- John (Truthful) No
- John (Liar) No
- Not John (Truthful - therefore John liar) Yes
- Not John (Liar - etc ...) Yes

GAME OVER

Will the real ellencatherine please stand up!

She's probably run off with John, after finding the real one.

:D

Will John Lie?

ok, i get it completely now.
Sorry to waste everyone time because i'm so stupid.
Thanks for the explanation, MASQUERAID.

this does work:
Does John Lie?

P.S.  Wwysdom, I'm a guy, but if i wasn't....
lol

Now to wait for ellencatherine to come back.

I was talking about our dear host, ellencatherine.

I know, this is probably one of those days, when you wake up, everything seems to have no logic, and you read everything wrongly.

Happened to me when I first read this question.

ok, yea monday has to be the worst day to make decisions.
Good thing I have philosophy classes on tuesdays.
: )

I'm baffled.  With whom does ellencatherine lie?  If it's John we should be told.

in respons to the twins one, they would both say yes becaues ones telling the truth and the other is lying

You may ask only one of the brothers a 3 word question.
So the only possible question is OZO's one:
Does John lie?
Cheers

dasna:
if you ask "does john lie"
then how do you know if you are talking to the liar of to the other one?

you only get and yes/no answer...

dasna:
if you ask "does john lie"
then how do you know whether you are talking to the liar of to the other one?

you only get and yes/no answer...

Let me try to explain why Ozo's solution works.  Let us call the other twin Tom, there are four possibilities:

Remember the question is "Does John lie?" and the riddle is to identify which twin is John.

1. John is asked the question and he is the liar so he answers "No"
2. John is asked the question and he is truthful so he answers "No"
3. Tom is asked the question and he is the liar (i.e. John is truthful) so he answer "Yes"
4. Tom is asked the question and he is truthful (i.e. John is the liar) so he answers "Yes"

So if you get the answer "No" you are talking to John else you are not talking to John.

Are you happy now ADPvvn?

ah well...

How to determine whether a very big number (suppose 9901313) is prime or not, for which our simple 2-20 prime number checking methods fails?

9901313 is prime

I concur with ozo, 9901313 is prime BUT this is way off topic!

how about using the question? u know u could make it a question like "is john blind" since they are identical, u can find the answer...

Quote:
"if you ask "does john lie"
then how do you know if you are talking to the liar of to the other one?"

This is precisely where I got hung up on this, and it took me a good 10 minutes KNOWING the answer to figure out the logic in it.

The question is NOT, "Is John the liar."  The question is, "Who are you talking to?"  You don't need to know if they're the liar or not.  Only which one you're speaking too.  If you ask the question "Does John Lie" of John, whether he's the liar or not, he'll always say "no."  If he's not a liar, he'll say "no, I don't lie."  If he IS the liar, he'll still answer "No, I don't lie."

If you ask the OTHER person the same question, and the OTHER person IS lying, then that means John DOESN'T lie, so the other person will say "Yes, John lies."  If the other person is telling the truth, then John, by default, is the liar.  So the other person will STILL say "Yes, John lies."

So you won't know if John is a liar or not, but that doesn't matter.  It wasn't the question.  John will always answer "no" and the other person will always answer "yes."

Great riddle.  It's actually a step ABOVE the question on Labyrinth.

-Javin

"Does John Lie?"
if No, then you have asked John.
If Yes, then John is the other one.

Dear nsstone,

Can you please read the responses before contributing.  You're answer is correct but your submission is three months after that answer has already been accepted.

The original question is an abbreviation of the original riddle. The original riddle goes like this: You are lost at a fork in the road and at that fork you encounter identical twins. One always lies and one always tells the truth. Both know the correct road to take you to the destination that you seek. What one (1) question can you ask one of the twins to set you on the right path? (Remember, you can only ask one question to only one of the twins and you do not know which one lies or which one tells the truth.)

Arghh! </subcribe>

I assumed (silly me) that since the question was STILL OPEN, answers were still being accepted.  My answer was both correct AND minimal, hence useful to those with less time on their hands than you obviously have.

Your comments should be directed to the guy who hasn't closed the question.

Regards,

nsstone

msblaster,

the answer to your sequence:

1, 11, 21, 1211, 111221, 312211

each number is a set of "pairs" describing the previous number:
1
"one 1"
"two 1",
"one 2, one 1",
etc

Don't assume people who follow threads from the beginning have a lot of time on their hands.  However, I would suggest you should spend a tad more time on these submissions to at least make sure you're on the right thread.

Lengore,

As the self-appointed Relevancemeister, please note msblaster's entry of 02/07.

Regards