The frequency (f ), wavelength (λ ) and speed (v ) of a sound wave...

The distance travelled by a wave in one time period $T$ is its wavelength $\lambda$.

The speed of the wave is given by $v = \dfrac{\text{Distance}}{\text{Time}} = \dfrac{\lambda}{T}$.

Since frequency $f = \dfrac{1}{T}$, substituting this into the equation gives $v = \lambda f$.

Answer: $v = \lambda f$