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)

Some of you have touched on this but here is how you have to think about it, put quite simply:

- If you are playing a game where the arbitrator flips two coins, looks at them, and then tells you "one them is a ______" and it can be assumed that if it's one of each, he randomly decides whether to say heads or tails, then the answer to the question is 1/2.

- However, if you are playing a game whereby the arbitrator will tell you that one of them is tails IF IT'S TRUE, and otherwise he won't tell you that, then you can assume it's 1/3 if he does tell you that one of them is a tails. Because under these guidelines, there was 1/4 chance that they are both heads and he can't tell you one of them is tails. 

Just saying that he DID tell you one is tails is actually different than saying that he COULD have honestly told you one was tails. It all depends on how you interpret the question. If this ever happened to me in real life - somebody walked up to me, flipped two coins, looked at them, and then told me that one is a tails and wanted to know what I thought the odds that the other one is also a tails, my honest-to-god answer to him would be "well, that depends, what game are we playing?"

Posted by Derek Hyatt on 2007-03-20 10:33:55