when does marginal revenue equal the market price
Marginal revenue For a firm facing perfect competition, price does not change with quantity sold (   P ′ ( q ) = 0  {\displaystyle P'(q)=0} ), so marginal revenue is equal to price. For a monopoly, the price decreases with quantity sold (   P ′ ( q ) < 0  {\displaystyle P'(q)<0} ), so marginal revenue is less than price (for positive   q  {\displaystyle q} ).