A 150 gallon tank is half full of pure water. A salt solution with a concentration 5 lb/gal is flowing into the tank at the rate of 4 gal/min while the well-mixed solution is flowing out at the rate of 1 gal/min. (a) Find Q(t), the amount of salt in lbs in the tank at time t minutes. (b) Find the concentration of salt in the tank when the tank begins to overflows.