The frequency of a wave is defined as the number of complete oscillations per unit of time:
[tex]f= \frac{N}{t} [/tex]
For the wave in this problem, we have 10 oscillations (because 10 crests pass through a given point in space) in a time of 120 s, therefore the frequency of the wave is
[tex]f= \frac{10}{120 s}=0.083 Hz [/tex]
