Solve all four parts

There are three industrial firms in the county. -Firm A's initial pollution level is 50 units. Its cost of reducing pollution by 1 unit is $30. -Firm B's initial pollution level is 40 units. Its cost of reducing pollution by 1 unit is $20. -Firm C's initial pollution level is 30 units. Its cost of reducing pollution by 1 unit is $40. The government wants to reduce pollution to 90 units so it gives each firm 30 tradable pollution permits. (i) Who sells permits? (ii) How many do they sell? (iii) Who buys permits? (iv) How many do they buy