In an airline hangar, jet-engines arrive at a rate of 2 per week. A jet engine costs nearly Rs. 1 crore. There are 30 aircraft, each with 4 engines. When an engine breaks down, it is sent for reconditioning, and the engine is immediately replaced by one from the spare engines’ bank of the reconditioned engines. If mean rate of reconditioning is 3 engines per week, what should be the optimum number of spare engines to be kept? Note that the cost of unavailability of a plane, if it is grounded, is Rs. 25 lakh per week.