An observer counts 4 complete water waves passing by the end of a dock every 10. seconds. what is the frequency of the waves?

frequency = 0.4 Hz

Explanation:
1- getting the period:
The period is defined as the time taken for one complete wave to pass. We are given that 4 complete waves pass in 10 seconds.
This means that:
4 waves > 10 sec
1 wave > ?? sec
period = 10/4 seconds = 2.5 seconds

2- getting the frequency:
frequency is the inverse of the period.
This means that:
frequency = 1/period = 1/2.5 = 0.4 Hz

Hope this helps :)