The engine of a railway train can cover a distance of 30 km in 20 minutes, when no weapons are attached to it. When wagons are attached, the decrease in speed is directly proportional to the cube root of the number of wagons attached. When 8 wagons are attached, it covers a distance of 14 km in 12 minutes. What is the speed of the train (in kmph) when 64 wagons are attached to the engine?