Please ASAP

Suppose that there are only two fishermen, Zach and Jacob, who fish along a certain coast. They would each benefit if a lighthouse were built along the coast where they fish. The marginal cost of building each additional lighthouse is $150. The marginal benefit to Zach of each additional lighthouse is 90 - Q, and the marginal benefit to Jacob is 40 - Q, where Q equals the number of lighthouses. What is the efficient number of lighthouses?