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 Actual Soluion by Drew)

Actually, Drew, you are confusing two questions. The probability that both coins are showing tails, given that one of them is (but nothing is specified regarding which one) is in fact 1/3. The probability that both coins are showing tails, given that a particular specified one (say the one on the left) is, however is 1/2. To illustrate the difference, imagine I throw a penny and a dime. You are given very different information (and should arrive at very different probabilities) depending on whether I tell you that "either the penny or the dime is tails" versus my telling you that "the dime is tails".

Posted by RoyCook on 2003-10-09 16:07:00