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)

For all of the people out there who STILL believe the answer is 1/2, let me ask you this:

Forget about the person saying anything at all. If two coins are flipped, what are the chances both are heads? Some of you have used the logic that tossing HT is the same as tossing TH. If that is the case then your answer to my question would be 1/3 instead of the correct 1/4. So clearly there is something lacking in that logic.

And to Charlie and anyone who agrees with him, did you even read the problem statement? It was written quite clearly. The person definitively tells you that one of the coins is a Tail, not "the person told you what one of the coins was." Your whole theory about a person's preference for announcing Heads or Tails is irrelevent. In this case we are dealing with a person who said Tails, period. All that did was eliminate the HH possibility.

Posted by nikki on 2003-06-25 12:18:47