The frequency of the wave is 0.05 Hz.
The frequency and period of a wave are inversely related to each other.
Given that the time period t is 20 seconds. The frequency f can be calculated as given below.
[tex]f = \dfrac {1}{t}[/tex]
[tex]f = \dfrac {1}{20}[/tex]
[tex]f = 0.05 \;\rm Hz[/tex]
Hence we can conclude that the frequency of the wave is 0.05 Hz.
To know more about the frequency and time period, follow the link given below.
https://brainly.com/question/13891370.