An international humanitarian organization has a fleet of 78 aid vehicles. The vehicles operate under challenging environmental conditions and are essential for providing services to communities in need around the World. Because of the extreme environmental factors under which they operate, the average useful life of a vehicle in this fleet is 39 months.

(a) What is the average number of vehicles that the organization must replace annually in order to maintain a constant fleet size?

(b) Because of the difficult operating conditions, the fleet requires a substantial amount of preventive maintenance and repairs. The fleet requires an average of 338 maintenance and repair events each year. At any given time, fleet vehicles that are not in maintenance and repair facilities are referred to as operational. What is the upper limit on the average time (in weeks) that vehicles can spend in maintenance and repair facilities to ensure a minimum average operational fleet size of 65 vehicles?