The Time period of a wave is measured as the wave divided by the frequency, while the Frequency is measured as the amount of waves divided by the Time Period. Therefore since we know that the Time Period of the wave is 20 seconds then we can calculate the following:
Frequency = 1 wave/ Time period
Frequency = 1 wave/ 20 seconds => simplify
Frequency = 0.05 wave/ sec
Finally we can see that the Frequency of the wave is 0.05 waves/sec or 0.05hz
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