please solve a and b clearly

1. (2 pts) I have 4 trucks for delivery of customer orders. One trip of a truck is needed for each order. The average time between customer orders is 5 hours. The average delivery time of a truck is 10 hours. Time between customer orders and delivery time (truck round trip time) follow exponential distributions. Use time unit of hours. a. Determine the probability that all trucks are working on delivery. b. Now, based on the JIT philosophy, my customers want quick delivery, and I wonder if more or fewer trucks than 4 would be helpful. The total cost for each truck (truck depreciation, insurance, driver wage, maintenance) per hour is 5 dollars. This cost incurs whether or not the truck is being used for delivery at a specific time. We do not consider other direct costs such as gas cost because this will be the same irrespective of the value of s. The customers want their order to be shipped (the products are loaded and the truck leaves the warehouse) quickly, and it is estimated that a customer's cost for waiting for one hour is $12. (If Table 6 on p. 1088 does not show the p values needed, use interpolation.)