7.23 A facility has four machines, with two repair workers to maintain them. Indi- vidual machines fail on average every 10 hours. It takes an individual repair worker on average 4 hours to x a machine. Repair and failure times are inde- pendent and exponentially distributed. (a) Find the generator matrix. (b) In the long term, how many machines are typically operational? (c) If all four machines are initially working, nd the probability that only two machines are working after 5 hours