The spring is attached to a fixed point in this problem. When the first pulse is created, it travels through the spring but once it reaches the end, due to action-reaction, the reflected pulse will be inverted or upside-down. The speed and frequency are just the same, but the amplitude (height) differs. Since some of the energy from the first pulse is transferred to the wall through vibrations, it comes back with lesser energy as evident in the much smaller amplitude. So when the reflected pulse with a height of >20 cm meets the second pulse at 20 cm, according to the Law of Conservation of Energy, the wave proceeds to be transmitted to the wall eliminating the reflected pulse. However, its amplitude will be less than 20 cm.
