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Truth and Lies

Braingle.com on

Published in Brain Teasers

There are three persons in a room.

One of them always tells the truth, the others are "alternate liars": they tell the truth then lie, then truth etc, but you do not know if they will start by telling the truth or a lie...

You can ask three questions that MUST be answered by Yes or No.

Find the truth teller.


Solution:

Q1. Mr. A, are you a truth teller?
- Yes
or
- No.

Q2. Mr. A, are you a truth teller?
- Yes
or
- No.

Q1: Q2:
Yes Yes: A is the truth teller
Yes No : A is an alternate liar, next answer is a lie.

Q3: Mr. A, is B the truth teller?
- Yes: C is the truth teller
- No: B is the truth teller
No Yes: A is an alternate liar, next answer is true.

Q3: Mr. A, is B the truth teller?
- Yes: B is the truth teller
- No: C is the truth teller
No No : impossible

Today's brain teaser courtesy of Braingle.com.


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