Maria helps tutor students taking economics. The equilibrium price for tutoring is $15 per hour

Maria has determined her opportunity cost per hour to be $6 for the first, $9 for the second, $12 for the third, $15 for the fourth, and $18 for the fifth. How many hours will Maria tutor? Why this amount of hours? What, if any, is Maria's producer surplus?

Maria will tutor for 4 hours. For all of the first 4 hours of tutoring, Maria's marginal cost of tutoring is less than or equal to the price she receives from tutoring. Maria will not tutor the fifth hour because for this hour her marginal cost exceeds the price. Maria makes a producer of surplus of $18, the sum of $9 on the first hour plus $6 on the second hour plus $3 on the third hour plus $0 on the fourth hour.