Speed = Wavelength x Frequency or V = f\lambda, where frequency = \(\frac{1}{\text{period}}\), f = \(\frac{1}{\text{T}}\). The period was given as 0.125sec and wavelength = 9cm : f = \(\frac{1}{\text{T}}\) = \(\frac{1}{0125}\) = 8. V = f\lambda = 8 \times 9 = 72m/s