The sound travels both ways.
2*distance=350*10
sound travels at 350 m/s in warm air. "If you hear the echo in 10 seconds, how far away is the reflecting surface?
2 answers
1,750 meters, since you're hearing the echo, and the ehco is twice the distance. So, you'd take 350 * 10 / 2 or just 350 * 5.