A total of 4 buses carrying 145 students from the same school arrive at a football stadium. The buses carry, respectively, 40, 30, 25 and 50 students. One of the students is randomly selected. Let X denote the number of students that were on the bus carrying the selected student. One of the 4 bus drivers is also randomly selected. Let Y denote the number of students on his/her bus.
a. Find the probability mass function of X and also that of Y.
b. Compute E(X) and E(Y ).
c. Compute V ar(X) and V ar(Y ).