the time between arrivals of buses follows an exponential distribution

Question

The time between arrivals of buses follows an exponential distribution, with a mean of 60 minutes. (a) What is the probability that exactly four buses will arrive during the next 2 hours? (c) That no buses will arrive during the next 2 hours? (d) A bus has just arrived. What is the probability that it will be between 30 and 90 minutes before the next bus arrives?