Each student in a certain high school was classified according to his year in school (freshman. sophomore. junior, or senior) =ld according to the number of times that he had visited a certain museum (never, once, or more than once). The proportions of students in the various classifications are given in the following table:
(a) If a student selected at random from the rugh school is a junior, what is the probability that he has never visited the museum?
(b) If a student at random from the high school has visited the museum three times, what is the probability that he is a senior?