A tank contains $50$ kg of salt and $2000$ L of water. A solution of a concentration $0.0125$ kg of salt per liter enters a tank at the rate $5$ L/min. The solution is mixed and drains from the tank at the same rate.

(a) What is the concentration of our solution in the tank initially?
concentration = (kg/L)

(b) Find the amount of salt in the tank after 4 hours.
amount = (kg)

(c) Find the concentration of salt in the solution in the tank as time approaches infinity.
concentration = (kg/L)