Question 263311
Why do you think that one of them can't or doesn't reply?
One always tells the truth.
One always lies,
and one randomly answers sometimes answers the truth and sometimes lies.
BTW Why do you address the question to "sir"? There are female tutors! and many  men don't like being called sir.
http://www.rule0.com/archives/148

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http://www.physicsforums.com/archive/index.php/t-36932.html