On a Typical Friday, an Average of 12 Customers Per

On a typical Friday, an average of 12 customers per hour arrive at the bank to transact business. There is one teller at the bank, and the average time required to transact business is four minutes. It is assumed that service times may be described by the exponential distribution. A single line would be used, and the customer at the front of the line would go to the first available bank teller. If a single teller is used, find:
a) The average time in the line.
b) The average number in the line.
c) The average time in the system.
d) The average number in the system.
e) The probability that the bank is empty.
f) If the business is considering adding a second teller (who would
work at the same rate as the first) to reduce the waiting time
for customers. She assumes that this will cut the waiting time in
half. If a second teller is added, find the new answers to parts (a)
to (e).