6. Arrivals to the express line of a grocery store follow a Poisson distribution with a mean rate of 20 per hour. Suppose that each transaction follows an exponential distribution with a mean of 2 minutes. In order to promote quality service, the grocery store claims to pay $5 to every customer who waits more than 5 minutes in the line. If the cost of opening each extra express line, which will split the arrival rates proportionally, is $7 an hour, how many express lines should be opened. (Assume that customers do not jockey in between queues).