A tank has capacity 6 L and initially contains 11 mg of salt dissolved in 3 L of water. A solution containing 1 mg/L of salt enters the tank at a rate of 3 L/hour and the well-stirred mixture leaves the tank at a rate of 2 L/hour. (a) Find the time when the tank is full. (b) Find the amount of salt (in milligrams) in the tank at any time before the tank is full. What is the amount of salt (in mg) in the tank when it is full and what is the concentration of the mixture then (in mg/L)?