The distance to the wall is 480m, so the distance travlled by the sound there and back is 960m. If the echo takes a total of 3s, then the speed of sound is approximately

\[v=\frac{distance \: travelled}{times \: taken}=\frac{960m}{3s}=320 m/s\]

.This is only an approximation. Probably the biggest error is in recording the time. A persons reaction time is accurate to about 0.3s, in pressing the button to start and stop, so a total possible error of 0.6s. This is equivalent to plus or minus 20% error in recording the time. The speed of sound is anyway not constant. If the temperature increases the speed of sound in air increase, because sound is transmitted by air molecules, which move faster.