Suppose a perfectly competitive firm and a monopolist are both charging $5 for their respective products. from this, one can infer that:

For the competitive firm marginal cost is $5. For the monopolist marginal cost is less than $5.

Explanation:

The price of the product of the competitive firm is $5. We know that a competitive firm is a price taker and produces at the point where the price is equal to the marginal cost of producing the last unit.  

A monopolist, on the other hand, is a price maker. It produces at the level of output where the price is greater than the marginal cost of producing the last unit.

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