We know that number of complete vibrations in one second is called frequency. Again the distance travelled by the wave during one complete oscillation is called wavelength.
So, if the wavelength is γ,
Therefore, the distance travelled in the time of 1 complete vibration = γ
So, the distance travelled in the time of f complete vibrations = fγ
If the time period is T, than in T second, the number of vibration is 1
Therefore, the number of vibration in one second is = 1/T
The number of vibration in 1 second is frequency. So, the frequency, f = 1/T
Since the frequency is f, so f numbers of waves are produced in 1 second.
So, the distance travelled in one second by the wave = fγ
This is the velocity of wave, v.
Therefore, velocity of wave, v = fγ.