An average of 9 cars can be seen in the system (both the drive-through line and the

Question:

An average of 9 cars can be seen in the system (both the drive-through line and the drive-through window) at Burger Universe. Approximately every 20 seconds, a car attempts to enter the drive-through line; however, 40% of cars simply leave the restaurant because they're discouraged by the length of the line. On average, how long does a car spend going through the drive-through at Burger Universe?