The temperature of the air is constant, 25°C, so the sound travels at its standard speed, c = 340 m/s.
The sound must travel back as well, so:
[tex]c = \frac{2\cdot D}{\triangle T}, \text{ where D = distance and } \triangle T \text{ is the elapsed time} [/tex]
[tex]D = (c \cdot \triangle T)\: /\:2[/tex]
[tex]D = (340\:m/s\:\cdot 5\:s)\:/\:2 = 1700 m / 2 = 850 m[/tex]
Hope it helps!
