Given,
Wavelength of the wave is λ
The period of the wave is T.
The distance is d.
The period of a wave is the time required by the wave to travel the distance of one wavelength. That is, in a time instant of T the wave will travel a distance of λ.
Therefore to travel a distance of d, the time required to the wave is given by
Thus it will take the wave (dT/λ) units of time to travel a distance of d