At one of its maintenance facilities, National Airlines stocks radar displays that are used in the cockpit of its CityHopper regional jets. While these displays are generally quite reliable, National has a large fleet of CityHoppers and on average, weekly demand for replacements is 1.3 displays, and demand follows a Poisson distribution. Lead time for displays is 8 weeks, and National uses a periodic review order-up-to policy with a review period of 1 week and an order-up to-level of 15 displays. a) Find the average on-order inventory for these displays. b) Find the average on-hand inventory for these displays. c) What is the stockout probability? d) Now suppose that National wants to maintain at least an in-stock probability of 0.915 while not having more on hand inventory than required to achieve that. What order-up-to level should they use? Please provide formula & explanation