An echo was heard after 3 seconds. How far away from the source is the reflecting surface if sound travels at a speed of 342 m/s?

Given, 
Speed of sound, v= 342 m/s
Time taken for hearing the echo, t = 3s 
Distance travelled by the sound: velocity x time
Distance = v x t = 342 x 3 = 1026 m 

An echo has to travel twice the distance between the cliff and the person. 
Hence, the distance between the cliff and the person$=\frac{1026 \mathrm{m}}{2}=513 \mathrm{m}$.

Therefore, 513 m is the correct response.