Answer
a). $(1)$
b). $100m$
c). $8.7s$
Work Step by Step
a). The answer is (1) , less than double, as the total time is sum of time taken for the stone to hit the ground and the time it takes sound to travel back that distance. While the time for sound is proportional to the distance, the time for free fall is not. (as $t= \sqrt (\frac{2d}{g})$.
Thus doubling the distance will only increase the free-fall time by a factor $\sqrt 2$.
b). time taken $= \sqrt (\frac{2d}{9.81})+\frac{d}{331+0.6(20)}=4.8s$
Therefore, $d=100m$
c). $t = \sqrt (\frac{2\times 300}{9.81})+\frac{300}{331+0.6(20)}=8.7s$
