A tank initially contains 200 gallons of fresh water but then a salt solution of unknown concentration

Question:

A tank initially contains 200 gallons of fresh water but then a salt solution of unknown concentration is poured into the tank at 2 gal / min. The well-stirred mixture flows out of the tank at the same rate. After 1 20 min, the concentration of salt in the tank is 1.4 lb/gal. What is the concentration of the entering brine?