Return to the situation described in Exercise 3.
a. Determine the marginal pmf of X1, and then calculate the expected number of customers in line at the express checkout.
b. Determine the marginal pmf of X2.
c. By inspection of the probabilities P(X1 = 4), P(X2 = 0), and P(X1 = 4, X2 = 0), are X1 and X2 independent random variables? Explain.