Application of Differential Equation: mixture problem

A 600 gallon brine tank is to be cleared by piping in pure water at 1 gal/min. , and allowing the well-stirred solution to flow out at the rate of 2 gal/min. If the tank initially contains 1500 pounds of salt, a) how much salt is left in the tank after 1 hour? b) after 9 hours and 59 min?