In a service centre, cars arrive according to Poisson distribution with a mean of two cars per hour. The time of servicing a car is exponential with a mean of 15 minutes. The expected waiting time (in minute) in the queue is A. 10 B. 15 C. 25 D. 30

The correct answer is A. 10.

The expected waiting time in the queue is equal to the reciprocal of the arrival rate times the service rate. The arrival rate is 2 cars per hour, which is equal to 1/30 cars per minute. The service rate is 1 car per 15 minutes, which is equal to 4/60 cars per minute. Therefore, the expected waiting time in the queue is 1/30 * 4/60 = 10 minutes.

Option B is incorrect because it is the mean service time. Option C is incorrect because it is the mean time between arrivals. Option D is incorrect because it is the mean of the sum of the service time and the waiting time.