6. A lake has three commercial resorts along with a public access area. All users of the lake benefit from improvements to water quality of the lake (still polluted from earlier industrial use). q is a measure of the water quality. Each commercial resort has demand for quality, Qc = 1000-5p and the public demand for quality is Qp = 1000-20p. The marginal cost of providing an additional unit of water quality is $90.