A splash is heard 3.91s after a stone is dropped into a well of 67.6 m deep. The speed of sound in air is

Time taken by the stone to reach the bottom of the well is $\mathrm{t}=\sqrt{\frac{2\mathrm{h}}{\mathrm{g}}}$

$\mathrm{t}=\sqrt{\frac{2\times 67.6}{9.8}}=3.71\mathrm{s}$

Time taken by sound to reach the top is 3.91-3.71=0.2s.

$\mathrm{v}=\frac{67.6 \mathrm{m}}{0.2 \mathrm{s}}=338\mathrm{m}/\mathrm{s}$