A tank contains $60$ kg of salt and $2000$ L of water. Pure water enters a tank at the rate $6$ L/min. The solution is mixed and drains from the tank at the rate $3$ L/min.

(a) What is the amount of salt in the tank initially?
amount = (kg)

(b) Find the amount of salt in the tank after 4.5 hours.
amount = (kg)

(c) Find the concentration of salt in the solution in the tank as time approaches infinity. (Assume your tank is large enough to hold all the solution.)
concentration = (kg/L)