Answer: Marginal Revenue = $18 per unit
Step-by-step explanation:
The profit maximizing condition for a firm in a monopolistic competitive market is at MR = MC.
So, the marginal revenue of the firm is equal to the marginal cost of the firm.
In this question, the firm's marginal cost is $18 per unit in the short run and it is a profit maximizing firm. Thus, its marginal revenue is $18 per unit.