A 'classic' prob stat question....
A bag holds three coins. One is double-headed and the other two are
fair. A coin is selected at random and flipped four times. What is the
probability that the coin is double-headed if it is observed that the coin
lands heads up on each of the four tosses?
This is from last Spring's final. See if you can get it.
(
http://www.math.gatech.edu/~belinfan.../pdf/final.pdf)