At an Automatic Car Wash, Cars Arrive Randomly at a Rate

Multiple Choice

At an automatic car wash, cars arrive randomly at a rate of 7 every 30 minutes.The car wash takes exactly 4 minutes (this is constant) .On average, how many customers would be at the car wash (waiting in line or being serviced) ?

A) 8.17
B) 7.46
C) 6.53
D) 0.46