The table below depicts the prices and total costs a local used-book store faces. The bookstore competes with a number of similar stores, but it capitalizes on its location and the word-of-mouth reputation of the coffee it serves to its customers. Calculate the store's total revenue, total profit, marginal revenue, and marginal cost at each level of output, beginning with the first unit. Based on marginal analysis, what is the approximate profit-maximizing level of output for this business?

Price per Book (S) 6.00 5.75 5.50 5.25 5.00 4.75 4.50 4.00 Output Total Costs (S) 2.00 5.25 7.50 9.60 12.10 15.80 20.00 24.75 4