We will consider two tanks. Tank A holds 80 gallons of water in which 25 lbs of salt are dissolved and Tank B contains 100 gallons of water. Pure water is poured into tank A at the constant rate of 4 gal/min. The well-mixed solution from tank A is constantly being pumped into tank B at a rate of 6 gal/min and the solution in tank B is constantly being pumped into tank A at a rate of 2 gal/min. The solution in tank B also exits the tank at a rate of 4 gal/min.

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