The speed of a wave is related to its frequency and its wavelength by
[tex]v=f \lambda[/tex]
where f is the frequency and [tex]\lambda[/tex] the wavelength. In our problem, [tex]f=0.40 Hz[/tex] and [tex]\lambda=420 m[/tex], therefore the speed of the wave is:
[tex]v=f \lambda=(0.40 Hz)(420 m)=168 m/s[/tex]
