Maintain the same general setup as before (competitive world market, price-taking firms). The world price of coal is a constant $45 per ton. The marginal private cost of coal production in the US is MC1.2Q. The external social cost of coal to the US is TSC0.9Q^2. This does not include the marginal private cost of production. If the government decides to solve the externality problem with tradable coal-mining permits, how many daily permits should it issue (it's ok to have fractional permits)