Final answer:
ABC Corp. should reduce its output since the average revenue per tonne is less than the marginal cost of the last tonne produced, indicating that the last unit is decreasing profits.
Step-by-step explanation:
If ABC Corp. is producing newsprint in a perfectly competitive industry at an output of 1500 tonnes per month with a marginal cost of $710 for the last tonne produced, and an average revenue per tonne of $620, then in the short run, this firm should consider its options based on the relationship between marginal cost, average revenue, and the price of the product.
Given that the average revenue, which is the same as the market price in perfect competition, is less than the marginal cost, it indicates that producing the last unit has added more to costs than to revenues. Therefore, the marginal unit is reducing profits, suggesting that the firm should reduce output. This strategy is aligned with the objective of a firm in perfect competition to produce at a level where marginal cost equals marginal revenue, which is the same as the price in such markets.