The disc containing the only copy of your term project just got corrupted, and the disc got mixed up with three other corrupted discs that were lying around. It is equally likely that any of the four discs holds the corrupted remains of your term project. Your computer expert friend offers to have a look, and you know from past experience that his probability of finding your term project from any disc is 0.4 (assuming the term project is there). Given that he searches on disc 1 but cannot find your thesis, what is the probability that your thesis is on disc i for i = 1,2,3,4.