The market demand in a Bertrand duopoly is P=15-4Q, and the marginal costs are $3

In oligopoly competition, the firms produce at the point where P = MC. For firms with different cost structures, the firm with the highest marginal cost will earn zero profits and the firm with the lowest marginal cost will tap all the industry profits. This is because the firm with the lowest marginal cost will set its price slightly below the marginal cost of the other firm. As a result, the firm will the smallest marginal cost will tap all the customers and earn all the industry profits.

If the firms have identical cost structures, the only equilibrium in the industry will occur at the point where the price is equal to the marginal cost for every firm (P = MC1 = MC2).

The marginal cost for each firm is MC = $3. Thus, each firm will set its price equal to P = MC = $3.