Problem $4.(5$ points$)$ Use the data from the drive$-$through example in Problem $3$ above. Because of limited space availability and a desire to provide an acceptable level of service, the bank management would like to ensure, with $95\%$ probability, that no more than three cars will be in the system at any one time.

a$)$ What is the present probability that $3$ or less cars will be in the system?

b$)$ If the teller could handle $25$ people per hour instead of $20,$ would that be sufficient to achieve the desired service level?