Answer:
Part a)
H = 26.8 m
Part b)
error = 7.18 %
Explanation:
Part a)
As the stone is dropped from height H then time taken by it to hit the floor is given as

now the sound will come back to the observer in the time

so we will have


so we have

solve above equation for H

Part b)
If sound reflection part is ignored then in that case



so here percentage error in height calculation is given as


And you asking how many meters can he run in 100 sec