A continuous feed agitated tank is initially filled with 500 liters of solution containing 31% salt. At t =0, the solution containing 4 kg/L salt is fed into the tank with a flow rate of 4 L/min, and the salt mixture is taken from the tank with a flow rate of 8 L/min. Assuming that the tank is well mixed, calculate the salt concentration (kg/L) in the tank after 39 minutes