A well-stirred tank contains 10000 kg of an aqueous solution of dilute methanol (xA=0.04 mass fraction methanol). A constant flow of 500 kg/hr of solution at a concentration of 0.07 is suddenly introduced into the tank and a constant rate of withdrawal of 400 kg/hr of solution is started. These two flows are continued and remain constant. Assuming that the volume of the tank is very large, calculate the time for the methanol concentration leaving the tank to drop to 1.0 wt%.