I don't understand how to solve this question: Company X is a firm that specializes in gardening. Last year, the firm had 30 employees and served 120 customers. This year, it had 35 employees and served 135 customers. The average product of labor (APL) is at its maximum when the number of customers is? (answer is: less than 120)

All I know is that the APL is at its max when it intersects the marginal product of labor (MPL).