Askipper on a boat notices wave crests passing the anchor chain every 5 seconds. the skipper estimates the distance between crests is 15 m. what is the speed of the water waves?

Because the crests occur every 5 seconds, the period is
T =  5 s.
By definition, the frequency is
f = 1/T = 1/5 = 0.2 Hz

The distance between crests is 15 m, therefore the wavelength is
λ = 15 m

By definition, the velocity, v, is
v = fλ
   = (0.2 1/s)*(15 m)
   = 3 m/s

  3 m/s

Uncle grandma