The town of Cleanville lies next to a lake, which the residents of the town use for fishing, boating, and other recreational activities. Last year, two firms, Filth Inc. and Sludge Inc., built factories on the other side of the lake and have been dumping trash into it. Although some of the trash will dissipate naturally, the amount of trash the two firms emit is too much for the lake to handle. Right now, each firm dumps 40 pounds of...