A reverse osmosis unit for purifying dilute salt solution has 900,000 hollow fibers that permit water and reject salt. The fibers have an outside diameter of 85 μm and an inside diameter of 42 μm. The average flow through the tubes is 2*106 cm3 in one day and the feed pressure is 3*106 N/m2. Calculate the pressure drop over a length of 3 m in each fiber. (Hint: each fiber is an annulus)