I will assume here that the well is sufficiently short so that the time the sound takes to come from the bottom of the well to our ear is negligible.
Since the pebble moves by uniformly accelerated motion, the distance it covers is given by
[tex]S= \frac{1}{2}gt^2 [/tex]
where
[tex]g=9.81 m/s^2[/tex] is the gravitational acceleration
[tex]t=2.7 s[/tex] is the time the pebble takes to reach the bottom of the well
Therefore, the depth of the well is
[tex]S= \frac{1}{2}(9.81 m/s^2)(2.7 s)^2 = 35.7 m \sim 36 m [/tex]
and the correct answer is B.
