A tank connected to two pipes (A and B) initially contains 300 gal of pure water. At time 0, a salt-water solution enters the tank through pipe A at the rate 4 gal/min, and pure water enters the tank through pipe B at the rate of 2 gal/min. Simultaneously, a drain is opened at the bottom of the tank, allowing the well-mixed solution to leave the tank at a rate of 3 gal/min.

(1) Suppose that the solution coming to the tank through pipe A contains 0.5 lb/gal of salt. What would be the amount of salt in the tank at the time t = 50 minutes?
(2) Suppose that the pipe A is closed at the time t = 50. How long would it take for the salt in the tank to be reduced to 1/2 of the value it had at time t = 50?