The sound emitted by the whistle travels across the distance L between the train and the reflecting surface, and then for another L to come back to the train. This means that the time t=5.0 s between the whistle and the echo corresponds to the time the sound takes to cover a total distance of 2L. Assuming the sound moves by uniform motion with speed v=343 m/s, we can write the basic relationship between distance covered, time and velocity:
[tex]S=vt=(343 m/s)(5.0 s)=1715 m[/tex]
As we said before, this distance corresponds to twice the distance between the train and the reflecting surface (S=2 L), so we find
[tex]L= \frac{S}{2} = \frac{1715 m}{2}=857.5 m [/tex]
and so, this is the distance between the train and the reflecting surface.
