Well, shame on Greg Mankiw for failing to cover the eternal battle between the adherents of Smith and Keynes in his 101 course. More interestingly, he also reposted this from FlowingData.

The answers to the blackboard question are better given as: a) 25% b) 50% c) This question is paradoxical and no definitive answer can be given. d) 25%

That way, there is no way out of the loop.

Mau

Cheeky indeed… You have 4 possible solutions, choosing randomly one would give you a 25% chance to be correct. But 25% appears two times as an answer, therefore, choosing randomly you have 2 possibilities of choosing the correct answer on 4 possible solutions, i.e. a 50% chance. But 50% being the correct solution, brings you back to the fact that one only is the correct out of 4…and you are back to 25%…….but….2dog is right…there is no way out the loop, being a paradox.