A teacher walks back and forth in front of the room during a lesson. He starts at one end of the desk and walk from the door to the window a distance of 4.0m in 5 s. Then he stops, turn around and walk 2.0 m back toward the door in 3 s. He stops here for 10 s and then walk 3 meters toward the door in 6 s. Deduce the average speed of the teacher over the entire 24 s interval?