Agirl floating in the ocean and watches 12 wave crests pass her in 46 s. calculate the wave frequency and period?

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

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:

It means that the girl sees 1 wave crest passing every 3.85 s in a certain point. 

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