The wavelength [tex]\lambda[/tex] of the wave is 160 m, and this is the distance between two consecutive crests. The boat is located at a crest of the wave, this means that the first trough is located 80 meters from the boat (because the distance between a crest and a trough is half the wavelength).
The speed of the wave is
[tex]v=56 km/h = 15.6 m/s[/tex]
so the time the boat takes to reach the first trough is
[tex]t= \frac{S}{v} = \frac{80 m}{15.6 m/s}=5.1 s [/tex]
