I toss two coins and look at the outcome. I then tell you that at least one of the coins is showing up as "tails". What is the chance that the other one is showing "tails" as well?

(from techInterview.org)

We don't have enough information to solve.  As stated, we don't know what prompted the narrator to declare that one of the coins is tails.  If he's simply reporting what the first coin shows, then the answer is 1/2.  The same would be true if he's simply reporting what the 2nd coin shows.  But if we know the narrator would say that one of the coins is tails if *either* coin actually is tails, then the answer is 1/3.

Of course, we haven't even considered the cases where the narrator may be lying... ;-)

 

Posted by Ken Haley on 2004-11-05 01:02:36