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