Question: Football Team Selection. A football team consists of 20 each freshmen and sophomores, 15 juniors and 10 seniors. Four players are selected at random to serve as captains. Find the probability that

a. All 4 are seniors

b. There is 1 each: freshman, sophomore, junior, and senior

c. There 2 sophomores and 2 freshmen

d. At least 1 of the students is a senior.