21. The Hi-Power Company provides electric power, supplied by four big generators, to an industrial area. The generators are subject to an occasional breakdown and, from the past data, it is known that a generator develops problem, on an average, once in every 12 months. The company has a repair crew which is entrusted with the task of repairing the faulty generators. From the past records, it is known that it takes 2 months, on an average, for a generator to be repaired. Calculate

(a) the proportion of time there shall be no generators to be repaired,

(b) the proportion of time there shall be one, two, three, and four defective generators,

(

c) the average number of generators waiting repairs,

(

d) the average number of generators out of service,

(

e) the average waiting time for repair of a defective generator to begin, and

(t) the average down-time per generator.