Assume citizens of Destrahan are complaining that Shell Chemical and Dow Chemical

Assume citizens of Destrahan are complaining that Shell Chemical and Dow Chemical are polluting the environment. Currently, each firm emits 40 tons of pollution per year. Shell Chemical could reduce its pollution at a cost of $500 per ton, and Dow Chemical could reduce its pollution at a cost of $100 per ton. To reduce overall pollution to 40 units, the mayor gives each firm 20 tradable pollution permits. Each permit allows a firm to emit one ton of pollution. Assuming each permit has a market value is $200, what is the cost of reducing pollution to the desired level