For a four-stage solvent extraction process, calculate the concentration of metal in the organic phase in the fourth stage given that the flow rate of aqueous solution is 1000 l/min, the organic flow rate is 500 l/min, the aqueous feed concentration is 6 g/l, the aqueous raffinate solution concentration is 0.1 g/l, and the initial concentration of metal in the organic feed solution is 0.5 g/l.