A 120-gallon tank initially contains 80 lbs. of salt dissolve in 90 gal of water. Brine containing 2lbs/gal of salt flows into the tank at the rate of 5 gal/min, and the mixture flows out of the tank...

A 120-gallon tank initially contains 80 lbs. of salt dissolve in 90 gal of water. Brine containing 2lbs/gal of salt flows into the tank at the rate of 5 gal/min, and the mixture flows out of the tank at the rate of 4 gal/min. If the tank is full after 40 minutes, how much salt does the tank contain?