Consider the simple table of marginal abatement costs (MCA) given to the right.

   If both firms are given credits to cover 50% of their current pollution, they must either reduce pollution by 50% or purchase credits. At 50% abatement, marginal cost for Firm 1 is 50 while for Firm 2 it is 110. Since it cost Firm 2 110 for the last unit of pollution reduced at 50%, if it can purchase credits for under 110 per unit it will be better off. Furthermore, since it only cost Firm 1 50 for the last unit it removed at 50%, if it can sell credits for more than 50 it will be better off.