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)

Number the coins. When you toss the two coins there are 4 equally likely events:
HH, HT, TH, TT
where they are arranged by their numbering.
by telling me that there is at least one tail, you only remove one of these events:
HT, TH, TT
Assuming that you are asking the probability that they are both tails,  the answer is 1/3.

Posted by owl on 2005-08-17 14:54:42