The frequency of a wave is the number of crests passing a point in one second.
If in t=46 s the number of crests passing a certain point is 12, the frequency will be
[tex]f= \frac{N}{t} = \frac{12}{46 s} =0.26 Hz[/tex]
So, 0.26 crests pass a certain point in 1 s.
The period of a wave is the time interval between two consecutive crests, and it is equal to the inverse of the frequency:
[tex]T= \frac{1}{f}= \frac{1}{0.26 Hz}=3.85 s [/tex]
It means that the girl sees 1 wave crest passing every 3.85 s in a certain point.
