For a perfectly competitive firm, marginal revenue (MR) is equal to the market price of the good. This is because, in a perfectly competitive market, firms are price takers; they can sell as much as they want at the market price, but they cannot influence the market price with their output. Therefore, any additional unit sold adds exactly the market price to total revenue, making MR equal to the price (P). Thus, we can express this relationship as:
\[ MR = P \]
This means that, for a perfectly competitive firm, each additional unit sold increases total revenue by the market price of that unit.