Ten small planes are flying at a speed of 150kmph in total darkness in an air space that  is  20×20×1.5km3 in volume. You are in one of the planes, flying at random within this  space, with no way of knowing where the other planes are. On an average, about  25×N  hours time will elapse between collision of a plane with your plane. Assume a  safety region as a sphere of radius 10 m around every plane. Find N.