7.3 A shop has five identical machines that break down independently of each other. The time until a machine breaks down is exponentially distributed with a mean of 10 h. There are two repairmen who fix the machines when they fail. The time to fix a machine when it fails is exponentially distributed with a mean of 3 h, and a failed machine can be repaired by either of the two repairmen. What is the probability that exactly one machine is operational at any one time?