Suppose a penny arcade worker is in charge of keeping ten machines in operation. The failure rate of each machine follows an exponential distribution with a mean time of ten hours and the time required to repair each machine follows an exponential distribution with a mean time of thirty minutes. Over the long run, approximately what percentage of machines, on average, will be operating?
a. 76.5%
b. 92.4%
c. 53.8%
d. 46.2%
e. None of the above