4. A certain store has an express cashier and a super-express cashier. Let X1 be the number of customers in the express cashier line. Let X2 be the number of cashiers in the super-express cashier line. Suppose that the joint pmf of X1 and X2 is Thus X1 takes on values 0,1,2 and 3 and X2 takes on values 0,1,2,3 and 4 . a. Find the probability that there is exactly one person in each line. (1 mark) b. Let A be the event that there are at least two more customers in one of the lines than the other. Find the probability of event A. (2 marks) c. What is the probability that the total number of customers in both lines is 4. (2 marks) d. If we know that there are two people in the express cashier line what is the probability that there are at least 3 people in the super express cashier line? (2 marks) e. Are X1 and X2 independent? Justify your response. ( 3 marks)