An entertainment magazine conducted an online poll of 230 college students who watch late-night TV, asking what shows they watch regularly. There were 119 who watch David Letterman, 101 who watch Conan O'Brien, and 75 who watch Jimmy Fallon. In addition, 69 watch Letterman and O'Brien, 48 watch Letterman and Fallon, 38 watch Fallon and O'Brien, and 28 watch all three.

a. How many don't watch any of these three shows?
b. How many watch only one of the three shows?
c. How many watch exactly two of the three shows?