A 200-gallon tank is half full of distilled water. At time t = 0, a solution containing 0.5 pound of concentrate per gallon enters the tank at the rate of 5 gallons per minute, and the well-stirred mixture is withdrawn at the rate of 3 gallons per minute. At what time will the tank be full?

Total volume of the tank=200 gallon
Given that, tank is half full,
so remaining volume of the tank to be filled=200-100=100 gallon.
Since 5 gallons of solution comes into the tank per minute and 3 gallons of solution goes out each minute,
Net flow of solution in the tank=5-3= 2 gallons per minute.
Time required=100/2=50
So, the time required for the tank be full = 50 minutes