Galvanized iron drainpipes of diameter 50 mm are located at the four corners of a building, but three of them become clogged with debris. Find the rate of downpour (cm/min) at which a single functioning drainpipe can no longer drain the roof. The building roof area is 500 m2, and the height is 5 m. Assume the drainpipes are the same height as the building and that both ends are open to the atmosphere. Ignore minor losses.