Oilseeds containing 30% by weight oil were extracted using pure hexane at constant pressure and temperature in a continuous and counter flow extraction unit. During the process, 1000 kg of hexane was used for 1500 kg of oilseed and a concentrated solution (upstream) containing 40% by weight of oil was obtained. Since it is known that 1 kg of non-oily solid material carries 0.52 kg of solution in the downstream leaving each stage in the extraction unit, how many stages did this process take?