The splash is heard 2.05S after the stone is dropped into a well of depth 19.6m The velocity of sound is $\left(g=9.8\mathrm{m}/{\mathrm{s}}^2\right)$

Given that depth of well  h=19.6m
Total time taken for both sound and stone is given that  t=2.05S
 Time taken by the stone to reach the water of well $t_1=\sqrt{\frac{2h}{g}}=\sqrt{\frac{2\times 19.8}{9.8}}$

$\begin{array}{c}=\sqrt{2\times 2}\\ =2S\end{array}$

$\therefore$Time taken by sound $t_2=t-t_1$

                                      $=2.05-2=0.05S$

$\therefore$Velocity of sound $=\frac{h}{t_2}$

                              $=\frac{19.6}{0.05}=392\mathrm{m}/\mathrm{s}$