Is my answer for part a correct? Is my idea for part b correct?... if not what should I do?

4][CHALLENGING BUT NOT IMPOSSIBLE] Assume four firms (A, B, C, and D) in the industry face the total abatement cost (TAC) and marginal abatement cost (MAC) schedules below. In the absence of any environmental policy, each firm in the industry will generate 10 units of pollution for an industry total of 40 units of pollution. Pollution TACA MACA TACB . MACC TACD MACD 10 20 30 40 9 18 27 36 8 110 90 72 56 42 30 16 24 32 55 45 36 28 21 15 10 6 3 7 14 0 1 2 3 4 5 6 7 8 9 10 165 135 108 84 63 45 30 18 9 21 28 6 12 18 220 180 144 112 84 60 40 24 12 4 0 24 5 20 10 15 20 4 8 12 16 3 6 9 12 12 6 2 0 2 4 6 8 0 1 2 0 3 4 a) What emission tax will encourage firms to abate pollution from the industry total of 40 units down to 20 units? Pollution tax of 5 b) How much has the industry saved in abatement cost by utilizing this tax (from part a) instead of utilizing a uniform emission standard that would require all firms to reduce pollution by 5 units. To find the answer do I take the difference of the TA for each firm from the pollution of 0-5? i.e. for firm Ca: 55-15= 40 and then add the forty to the money saved from firm Cb, Cc, and Cd? I am very confused on this problem 4][CHALLENGING BUT NOT IMPOSSIBLE] Assume four firms (A, B, C, and D) in the industry face the total abatement cost (TAC) and marginal abatement cost (MAC) schedules below. In the absence of any environmental policy, each firm in the industry will generate 10 units of pollution for an industry total of 40 units of pollution. Pollution TACA MACA TACB . MACC TACD MACD 10 20 30 40 9 18 27 36 8 110 90 72 56 42 30 16 24 32 55 45 36 28 21 15 10 6 3 7 14 0 1 2 3 4 5 6 7 8 9 10 165 135 108 84 63 45 30 18 9 21 28 6 12 18 220 180 144 112 84 60 40 24 12 4 0 24 5 20 10 15 20 4 8 12 16 3 6 9 12 12 6 2 0 2 4 6 8 0 1 2 0 3 4 a) What emission tax will encourage firms to abate pollution from the industry total of 40 units down to 20 units? Pollution tax of 5 b) How much has the industry saved in abatement cost by utilizing this tax (from part a) instead of utilizing a uniform emission standard that would require all firms to reduce pollution by 5 units. To find the answer do I take the difference of the TA for each firm from the pollution of 0-5? i.e. for firm Ca: 55-15= 40 and then add the forty to the money saved from firm Cb, Cc, and Cd? I am very confused on this