Awave has a wavelength of 13 mm and a frequency of 4 hertz. what is its speed?

The velocity of a wave is described by the following equation:

Where v is velocity, λ is the wavelength and f is the frequency.  Note that the wavelength given is:

Thus,

The velocity is 5.2 cm/second or 0.052 m/s or 52 mm/s

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.