A graph shows Quantity on the horizontal axis and Price and Cost in dollars on the vertical axis. The horizontal axis ranges from 0 to 80 with an interval of 10. The vertical axis ranges from 0 to 8 with an interval of 1. The graph shows a horizontal demand curve, d, with a vertical intercept of (0, 6) and extending just beyond a quantity of 80 units. The graph also shows an average total cost curve, ATC, which is initially downward sloping then becomes upward sloping for quantities greater than 50 units. The following coordinates are noted on ATC: ? (30, 6) ? (50, 4.50) ? (70, 6). A marginal cost curve, MC, is plotted on the graph. The MC curve is downward sloping for quantities up to 30 units, then upward sloping for quantities greater than 30 units. The following coordinates are noted on MC: ? (10, 6) ? (30, 2) ? (50, 4.50) ? (60, 6). Refer to Figure 22-4. The perfectly competitive, profit-maximizing firm will produce __________ units of output