What is the period of a wave traviling 5 ms it its wavelength is 20 m?

If a wave is traveling at 5 meters per second (assuming that is what the question meant) and its wavelength is 20 meters, consider standing beside the wave and watching it pass. As the wave is 20 meters long and it is moving at 5 meters per second, it will take 4 seconds for the full cycle of the wave to pass an observer. That means its frequency is one cycle per 4 seconds. And - surprise! - that's the period of the wave. The period of the wave is 4 seconds.