A certain market has both an express checkout line and a super-express check out line. Let X1 denote the number of customers in line at the express checkout at a

particular time of day, and let X2 denote the number of customers in line at the super-express checkout line at the same time. Suppose the joint pmf of X1 and X2 is given below: What is the probability that there is 1 person in the express checkout line?