There is a large mixing tank that initially holds 80gallons of brine (water in which salt has been dissolved). Another brine solution is pumped into the tank at a rate of 2gallons per minute. The tank keeps the mixture well-stirred, and the solution is pumped out of the tank at the same rate of 2gallons per minute. If the solution being pumped into the tank has a salt concentration of 4lb/gal, and the tank initially contained 80 lb of salt. Required:
Find how much salt will be in the tank after 20 minutes.