Drexel dragonThe Math ForumDonate to the Math Forum

Ask Dr. Math - Questions and Answers from our Archives
_____________________________________________
Associated Topics || Dr. Math Home || Search Dr. Math
_____________________________________________

Who is a Liar, Who Tells the Truth?


Date: 9/4/96 at 19:7:34
From: Anonymous
Subject: Who is a Liar, who Tells the Truth

The inhabitants of a certain island have a strange characteristic: 
those who are swimmers always tell the truth, and those who are non-
swimmers always lie.  

You visit the island and meet a group of three native inhabitants. 
You ask the first person, "Are you a swimmer?" The answer is 
unintelligible. The second person immediately says, "That was 'yes.' 
I am a swimmer, too." The third person says, "They are both lying." 

Is each of these three people a swimmer or a nonswimmer?  

My answer: The first is a nonswimmer, the second is also a nonswimmer, 
the third one is a swimmer.


Date: 9/4/96 at 21:54:41
From: Doctor Robert
Subject: Re: Who is a Liar, who Tells the Truth

Put yourself in the place of the second of the three natives.  If the 
first person is a swimmer he will say "I am a swimmer."  If the 
first person is a non-swimmer, he will also say "I am a swimmer" 
(because he always lies).  So, the thing that the second person hears 
is "I am a swimmer."  

When the second person says that the first person says he is a 
swimmer, the second person is telling the truth and is therefore a 
swimmer.  

Now, the third person says that the second person is lying - which is 
wrong.  So that makes the third person a non-swimmer.  Unfortunately, 
we do not know whether the first person is a swimmer or non-swimmer.  
Using S for swimmer and N for non-swimmer, the possibilities are

   SSN   or NSN  
    
[See also the Classic Problem "Liars and Truthtellers" in the
Dr. Math FAQ: 
 
   http://mathforum.org/dr.math/faq/faq.liar.html   .]

-Doctor Robert,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/   
    
Associated Topics:
High School Logic

Search the Dr. Math Library:


Find items containing (put spaces between keywords):
 
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || Math Library || Quick Reference || Math Forum Search
_____________________________________

Ask Dr. MathTM
© 1994-2013 The Math Forum
http://mathforum.org/dr.math/