perplexus.info :: Logic : Bob and his brother, revisited

Bob and his brother, revisited (Posted on 2002-04-24)

You are to meet Bob and his brother again. Imagine you still don't know which one of them is Bob, and wether or not Bob always lies or tells the truth. In other words, let's say you never solved the first problem.

But this time, your task is to find out wether or not Bob, whichever brother he is, will lie to you.

It is possible to find this out with a short question to either one of the brothers. (A question consisting of four to five words.) What will you ask?

Submitted by levik

Rating: 3.1818 (11 votes)

Solution: (Hide)

A mirror image of the original Bob and his brother problem, the question to ask in this case is "Are you Bob?.
If the person you ask is

Bob, and truthful, he will say "yes"
Bob, and a liar, he will say "no"
Not Bob and truthful, he will say "no" (in this case Bob is a liar)
Not Bob and a liar, he will say "yes" (in this case Bob is truthful)
As you see, a "yes" answer means Bob is truthful, and "no" means he is a liar.

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

Subject	Author	Date
anwser	Sean	2005-10-27 11:01:58
my answer	ubergeek	2002-12-25 23:24:04
re: the answer	levik	2002-08-27 05:59:20
the answer	Kristina	2002-08-26 12:39:24
re: this is just like Labyrynth	levik	2002-08-26 09:50:17
this is just like Labyrynth	Mel	2002-08-26 01:33:18

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