How do I figure out number 4?

Total Marginal Total Marginal Average Total Price Demanded Revenue Revenue Cost Cost Cost $ 20 0 $ 18 12 $ 16 20 $ 14 30 $ 12 42 $ 10 56 $ 8 6 72 a. At what rate of output is total revenue maximized in this range? b. What are the values of marginal revenue and marginal cost at the revenue-maximizing rate of output? C. At what rate of output are profits maximized within this range? d. What are the values of marginal revenue and marginal cost at the profit-maximizing rate of output? Focus O e 43 O Id Ai DELL