Suppose we have a tank containing 10 gallons of fresh water. Water containing 4 lbs/gal of salt enters at a rate in gal/min equal to half the current volume of water in the tank, and the well-stirred mixture leaves the tank at a rate in gal/min equal to a third of the current volume of water in the tank. If the tank has capacity 20 gallons, how much salt is in the tank at the moment it overflows?