4 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?

b That at least two 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?