Answer:
The time elapses until the boat is first at the trough of a wave is 4.46 seconds.
Step-by-step explanation:
Speed of the wave, v = 59 km/h = 16.38 m/s
Wavelength of the wave,

If f is the frequency of the wave. The frequency of a wave is given by :

The time period of the wave is given by :

We need to find the time elapses until the boat is first at the trough of a wave. So, the time will be half of the time period of the wave.

Hence, this is the required solution.