Answer:
2 Hz
Step-by-step explanation:
The frequency of a wave is equal to the reciprocal of its period:

where T is the period of the wave.
The period of a wave corresponds to the time it takes for one complete oscillation of the wave to pass a given point. In this problem, one complete wave passes the pile in 0.50 s, so the period of the wave is T = 0.50 s. Therefore, the frequency is:
