The ratio of change in total revenue to a small change in quantity, MR = ΔTR / ΔQ, is defined as marginal revenue, above. Similarly, marginal cost is defined as the ratio of change in total cost to a small change in quantity, MC = ΔTC / ΔQ If the firm is operating in a perfectly competitive market, then the price is given from the firm’s perspective. The more produced and sold, the higher the total revenue in this case.