19 (1 point) At a fashion retailer, there are usually multiple cashiers providing checkout service to customers simultaneously. They are paid at the hourly rate of $20. On average, customers arrive at the checkout area every 6 minutes. It is estimated that the customer arrival process is a Poisson process. The average checkout time for each customer is 12 minutes, with its standard deviation equal to 12 minutes. Suppose that customers form a single line. The manager estimates that keep a customer waiting for each hour leads to the penalty cost of $35 (due to loss of goodwill, low rating on review websites, etc.) What is the optimal number of cashiers required, in order to minimize the total cost (payroll cost + customer waiting cost) per hour? Note: 1. Either use Excel for your calculation, or keep at least 4 decimal places for your intermediate numbers. 2. The Poisson arrival process has exponentially distributed inter-arrival times. Your