An echo is returned in 3 seconds. What is the distance of the reflecting surface from the source given that the speed of the sound is 342 m/sec?

When a sound is reflected by a surface and reflected back to the source of the sound, it is said to be an echo. 

So, according to the question, the echo travels 2d distance in 3s with a speed of 342 m/s. 

Therefore, the required distance
2d = v $\times$ t = 342 x 3 = 1026
d = $\frac{1026}{2}$ = 513 m
Thus, the distance of the reflecting surface from the source is 513 m.