A 1200-gallon tank initially contains 40 pounds of salt dissolved in 600 gallons of water. Starting at 1 = 0, water that contains 0.5 pounds of salt per gallon is added to the tank at a rate of 6 gallons per minute and the resulting mixture is drained from the tank at a rate of 4 gallons per minute. (a) Find the quantity A(t) of salt in the tank at any time I prior to the overflow. (b) Find the amount of salt A* in the tank at the time t* of overflow. (c) What is the concentration of salt in the tank at the time (*