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)

(In reply to It's not 1/3 the way the problem was posed. by Charlie)

Charlie, I believe the logic of your answer is flawed, because it assumes that you will have no preference for what you report, and therefore you will report "at least one head" during one of the two outcomes in which you get one head and one tail.

But this is not a puzzle about preferences, it is a puzzle that asks, given the report of "at least one tail", how many ways can there be at least one tail from two coins tossed together, and of those ways, how many involve both coins being tails.

Trying to read into the psyche of the puzzle's narrator introduces artificial constraints on the problem, resulting in a distorted answer. Assuming nothing that is not given in the problem statement, the answer is 1/3.

Posted by Bryan on 2003-03-26 12:02:29