A fast food restaurant has a single line for customers to us

A fast food restaurant has a single line for customers to use while waiting for the next available server. There are two servers 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 four per minute for each of the servers. What is the average time a customer spends in the system?

a. 0.6431
b. 0.7250
c. 0.1907
d. 0.2909
e. none of these

Solution

lambda = 3 , u = 4

average time = lambda/u(u-lambda) = 3/(4* (4-3)) = 3/4 = 0.75

so, correct choice is e) none of these