Use Table: Variable Costs for Lawns. During the summer, Alex runs a lawn-mowing service, and lawn-mowing is a perfectly competitive industry. Assume that costs are constant in each interval; so, for example, the marginal cost of mowing each of the lawns from 1 through 10 is $10. Also assume that he can only mow the quantities of lawn given in the table (and not numbers in between). His only fixed cost is $1,000 for the mower. His variable costs include fuel, his time, and mower parts. If the price for mowing a lawn is $40, how much is Alex's profit per unit at the profit-maximizing output?

Table: Variable Costs for Lawn Table: Variable Costs for Lawns Quantity Variable Costs of Lawns 0 $0 10 100 20 300 30 500 40 1,100 50 1,800 60 2,900