A Pharmacy has four counter to serve Blood pressure test for customer. Customers arrive according to a Poisson distribution at the rate of 1 every 10 minutes. However, only 85% seek service at the counter. The service time per customer is exponential, with a mean of 10 minutes. All arrival customers form one line and access available windows on an FCFS basis.

a. What is the probability that an arriving customer will wait in line?

b. What is the probability that both windows are idle?

c. What is the average length of the waiting line?

d. Would it be possible to offer reasonable service with only three counter? Explain!