Apost office has a single line for customers waiting for the next available postal clerk. there are two postal clerks who work at the same rate. the arrival rate of customers follows a poisson distribution, while the service time follows an exponential distribution. the average arrival rate is three per minute and the average service rate is two per minute for each of the two clerks. what is the average length of the line? 3.429 1.929 1.143 0.643 none of these