In the figure shown, an observer $O_1$ floats (static) on water surface with ears in air while another observer  $O_2$ is moving upwards with constant velocity  $V_1=V/5$  in water. The source moves down with constant velocity  $V_2=V/5$ and emits sound of frequency  $f$. The velocity of sound in air is V and that in water is 4V. For this situation the wavelength of the sound received by  $O_2$ is  $\frac{16V}{xf}$. Find x