A 60-gallon tank initially contains 30 gallons of sugar water, which contains 12 pounds of sugar. Suppose sugar water which containing 2 pound of sugar per gallon is pumped into the top of the tank at a rate of 4 gallons per minute. At the same time, a well-mixed solution leaves the bottom of the tank at a rate of 2 gallons per minute. How many pounds of sugar is in the tank when the tank is full of the solution?