Q14. A total of 60,000 hours have passed since a computer system was put into operation. During this operation period, a total of 100 failures occurred, and the system was repaired each time. The last repair was completed at the 60,000th hours. If the MTTR is 60 hours, how many hours is the MTBF of the system during this period?

a) 480 b) 540 c) 599.4 d) 600

the Ans. is b) 540 but Please show me how to calculate.