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 I think the solution is WRONG by Lewis)

Whoops - forgot to explain this.

In the solution, it says that there are three possible outcomes if the first coin is heads - HT, TH & TT.
HT is not possible, because the first coin was T, not H.
But if the order doesn't matter, then it is exactly the same as TH. So really the only two possible outcomes are TT and TH, so the probality of the second coin being tails is 1/2

Posted by Lewis on 2003-05-30 06:54:40