Step-by-step explanation:
By using Ms Excel you should do this problem.
Kindly check the attachment for solution and cell reference for the clear understanding.
To get your answer you need to get the average the plane goes and then multiply it by 35 hours. hope this helps:)
3/8 = 0.375 hrs per wax
60 x 0.375 = 22.5 hrs for 60 cars