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)

Surely the answer is not 1/3, but 1/2? Really, what one coin shows will have absolutely no effect on what the other coin is showing? Its like tossing a coin 99 times and getting tails each time, you can't then say "Its more likely I'm going to get a heads than tails this time". They're completely independant events, so unless the narrator has certain preferences as to what to say in certain situations, (which is not stated in the question), the answer will be 1/2.

Posted by Matt on 2007-10-23 13:45:31