at the order fulfillmdnt center of major mail order firm customer orders already pacaged for shipment areive at a sorting maxhibe to be sorted for loading onto the approperate truck for thr parcels address. the arrival rate at thr sorting maching is 90 packages ler hour following a poisson distributuon. thr maxhine sorts at thr constnt raye of 120 pacages per hour the average number kf paczgds waiting in the hopper to be sorted is