perplexus.info :: Logic : Bob and his brother

Bob and his brother (Posted on 2002-04-22)

You are to meet with two brothers. You know that one of them, named Bob, has a very important package for you. Unfortunately, you never saw Bob or his brother, and don't know who is who.

To make the matter worse, one of the brothers is a pathological liar who never tells the truth, while the other never lies. Not only do you not know which is the liar and which tells the truth, but also, you do not know wether Bob is the liar or not.

Upon meeting the brothers, you can ask one short question of either of the brothers (but not both) to establish with certainty which of the two is Bob. (It is possible to do this with a question of four to five words.)

What question would you ask?

Submitted by levik

Rating: 3.5882 (17 votes)

Solution: (Hide)

Ask one of them "Is Bob a liar?".
There are four possibilities:

The brother is Bob and tells the truth: He will answer "No"
The brother is Bob and is a liar: He will lie and answer "No"
The brother is not Bob and tells the truth: He will answer "Yes"
The brother is not Bob and is a liar He will lie and say that Bob (who tells the truth) is a liar.
As you can see, when asked this question, any brother would say "No" if he is Bob, and say "Yes" if he is not.
Similarly, you can ask if Bob tells the truth, and reverse the answers' meanings.

Comments: ( You must be logged in to post comments.)

Subject	Author	Date
Puzzle Solution: Identifying Bob's Type	K Sengupta	2022-02-27 00:06:40
Puzzle Solution	K Sengupta	2022-02-26 23:51:25
solution	Minerva	2007-01-28 14:57:41
another solution	salil	2006-02-25 06:01:41
Thoughts	salil	2006-02-25 05:55:49
re: If Bob is the liar	Ken Haley	2004-09-22 00:01:19
If Bob is the liar	Ryan	2004-05-03 12:45:19
No Subject	Pieater	2004-03-05 12:05:19
re: just read the solution	JennaBanenna	2003-10-29 00:51:29
just read the solution	JennaBanenna	2003-10-29 00:48:02
re: solution	Sam	2003-09-22 00:34:38
solution	Daniel	2003-08-12 05:37:18
OH!	zineera	2003-05-26 14:10:43
A long pondered question	Cara	2003-05-08 10:30:11
3 word question	Ken	2002-09-11 20:26:22
re: the answer	levik	2002-08-27 03:47:57
the answer	Kristina	2002-08-26 12:41:58
re: (After reading the solution)	TomM	2002-08-04 03:47:29
(After reading the solution)	Hazirah	2002-08-03 23:26:31
Ai... yaah!	Hazirah	2002-08-03 23:19:39
try this too	Carey	2002-05-28 00:18:37
no genius necessary	Mike	2002-05-10 15:48:19
Solution	Max Proskoff	2002-04-23 12:00:19

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