Final answer:
The total revenues for the purely competitive firm producing 30 units at a marginal revenue of $12 per unit would be $360.
Step-by-step explanation:
The question pertains to the total revenues for a purely competitive firm that earns a marginal revenue of $12 for each additional unit sold and is producing 30 units. If the firm earns a marginal revenue of $12 for each additional unit it sells, and it is currently selling 30 units, then to calculate total revenue, you would multiply the number of units sold by the marginal revenue per unit (assuming marginal revenue equals price). Therefore, total revenue would be the product of 30 units and $12 per unit.
However, the information given in the examples talks about a different scenario involving a firm producing five units at $25 per unit and facing different costs. This information is not relevant to the student's question and will not be used to calculate the total revenue for the firm producing 30 units at a marginal revenue of $12 per unit. So based on the student's question, the firm's total revenues when selling 30 units would be $360 (30 units x $12/unit).