Given,
There are 40 crests and 40 troughs.
One crest and one trough constitute one cycle.
So, the total number of cycles = 40
One cycle denotes one time period.
Time Period = \(\frac{Total\,time}{No.\, of\, cycles}\) = \(\frac{0.4}{40}\) = 0.01s
Frequency = \(\frac{1}{Time\,Period}\) = \(\frac{1}{0.01}\) = 100 Hz