Having trouble with this one:

The customer service counter at the bank has a mean of 5 customers arriving in a given hour, and the number of customers arrviing in an hour is known to be distributed as a poisson random variable

a) what is the probability that the time between customers arriving is greater than a half hour

b)what is the prob that the time btwn customers arriving is between 15 and 45 minutes

Thanks for any help