An air-traffic controller observes two aircraft on his radar screen. The first is at altitude 900 m, horizontal distance 19.9 km, and 25.0° south of west. The second aircraft is at altitude 1300 m, horizontal distance 18.0 km, and 20.0° south of west. What is the distance between the two aircraft? (Place the x axis west, the y axis south, and the z axis vertical.)