Answer:
The car that arrives at 10 AM will have to wait for at least 1 hour before it is washed.
Explanation:
First of all, let us lay out the important points:
at 9 AM; 10 cars were already waiting
arrival rate = 20 cars per hour
wash rate = 15 cars per hour.
New car arrives at 10 AM
From the statements above, if 20 cars arrive every hour, it can be inferred that between 9 AM to 10 AM, the number of new cars that arrive = 20 cars.
But remember that before the car was opened, 10 cars were already waiting in line, therefore the total number of cars that arrived between 9 AM and 10 AM = 20 + 10 = 30 cars.
We are also told that the rate of wash of the cars are; 15 cars every hour. Therefore between 9 AM and 10 AM which is 1 hour, 15 out of 30 cars were washed, leaving 15 more cars unwashed. To wash this 15 remaining cars, another hour will pass, so by 11 AM, the cars that arrived between 9 AM and 10 AM will be completely washed. hence the car that arrived at 10 AM, will wait till 11 AM in order to be washed, which is a wait time of 1 hour.
