Question: Midterm 2 of CS70 consists of n questions. Before the test, m TAs are going to beta test the midterm. Each of them will solve a question correctly with probability p (independently of other TAs and independently of other questions). Let X be the number of distinct questions that no one solves correctly.
(a) What is the expectation of X? What's the variance of X?
(b) Now each TA is going to choose a question uniformly at random from the n questions to grade (independently of other TAs). Let Y be the number of distinct questions that no one chooses. What is the expectation of Y? What's the variance of Y?