3. Consider a class of 25 students: 15 are juniors and 10 are seniors. Sup- pose we randomly choose a group of 4 students to give a presentation. Let X be the number of seniors in the group. (a) Find the pmf of X. (b) Find the cdf of X. (c) What is the probability of exactly 1 senior being in the group? (d) What is the probability the group is all juniors? (e) What is the average number of seniors we can expect to be in the group