The Post Office Uses a Multiple Channel Queue, Where Customers

The post office uses a multiple channel queue, where customers wait in a single line for the first available window. If the average service time is 1 minute and the arrival rate is 7 customers every five minutes, find, when two service windows are open,
a.the probability both windows are idle.
b.the probability a customer will have to wait.
c.the average time a customer is in line.
d.the average time a customer is in the post office.