A business has 5 telephone lines. The first rings every minute on the average, the second every ½ minute, the third every 2 minutes, the fourth every 5 minutes, and the fifth every 1/10 minute, on the average. Let Xt = #(calls in [0,t]).
(a) What is the exact distribution of Xt? Find (b) E(X60) and (c) P(X60 < 850) using the normal approximation.