In a multi-stage counter-current flow leaching process, a solid material at a flow rate of 1000kg/h containing 10% oil (A) and 5% hexane (C) is extracted. 300kg/h pure hexane is fed from the overflow and used to extract 50% of the oil in the material. If the final underflow solution includes 0.50kg oil / kg hexane, what is the flow rate of the solution (V1) leaving from the last overflow stage