The period of a wave can be calculate according T = 1 / f where T is wave period and f is frequency, then in our problem we have a wave of 100 Hz so: T = 1 / 100 = 0.01 seconds The wave have a period of 0.01 seconds. Optinaly we could also calculate the speed: V=wavelenght/period= 2/0.01= 200 m/s