Mary's Fence Post Factory faces a perfectly elastic demand curve for fence posts at a price of $39 per post. Let Q represent the number of fence posts that Mary makes. Mary's total cost and marginal cost curves for making fence posts are:
TC = 4,000 + 3Q + 0.1Q2
MC = 3 + 0.2Q
Graph Mary's marginal cost curve using the orange line (square points) and her marginal revenue curve using the blue line (circle points). Place a green point (triangle symbol) at the price and quantity at which Mary would maximize short-run profits.