Pls tell ans of this ques.. ch is units and measurement

Solution,
Time taken by the stone to reach the bottom of the well
$t_1=\sqrt{\frac{2L}{g}}$
Time taken by the sound to reach the observer from the bottom
$t_2=\frac{L}{v}$
Total time interval,
 $$\begin{gathered} t=t_1+t_2=\sqrt{\frac{2L}{g}}+\frac{L}{v} \\ \delta t=\frac{1}{2}\sqrt{\frac{2}{g}}\frac{\delta L}{L}+\frac{\delta L}{v} \\ \delta t=\delta L\sqrt{\frac{1}{2Lg}}+\frac{\delta L}{v} \\ \delta t=\left(\sqrt{\frac{1}{2Lg}}+\frac{1}{v}\right)\delta L=\left(\sqrt{\frac{1}{2\times 20\times 10}}+\frac{1}{300}\right)\delta L=\frac{4}{75}\delta L \\ So, \\ \delta L=\frac{75}{4}\times \delta t=\frac{75}{400} \\ \frac{\delta L}{L}\%=\frac{75}{400\times 20}\times 100\%=0.9375\%=1\% \end{gathered}$$