A tank holds 60L of water-salt solution in which 4kg of salt is dissolved. Water flow into the tank at the rate of 3L/min and salt solution overflows at the same rate. If the mixing in the tank is adequate to keep the concentration of salt in the tank uniform at all time, how much salt is in the tank at the end of 60min. Assume that the density of the salt solution is essentially the same as that of water. [20 marks]