A cistern in the form of an inverted circular cone is being filled with water at the rate of 55 liters per minute. If the cistern is 7 meters deep, and the radius of its opening is 5 meters, find the rate at which the water level is rising in the cistern 20 minutes after the filling process began. Round any intermediate calculations, if needed, to no less than six decimal places, and round your final answer to three decimal places. (Hint: 1⁢m^3=1000⁢L.)