Soybeans have a composition of 72% solids, 18% oil and 10% water, and are ground to extract the oil. The oil extraction is carried out with hexane, and a mixture of 25% oil and 75% hexane is obtained. The residue contains 0.5% oil, 20% hexane, 9.7% water and the rest solids. How many lbs of soybeans must be fed if 9000 lbs of the main mixture are obtained?