A boy fires a gun and hears the echo 2 seconds later, if he is 350 m away the velocity of sound in air.

Hint: Echo – If any sound source emits the sound and collision to object, after just that sound return in their medium, hence the returnable sound is called echo. Generally the minimum distance between an object and a human's ear should be 17 meter to hear echo.

Complete Step by step solution
Given data, Distance between boy and object, d = 350 meter; Hence the total distance travelled for echo = 2d
Taken time by echo, t = 2 second
Velocity of sound, v =?
Hence, according the formula of distance;
Distance = Velocity x Time
 $ \Rightarrow {d = vt} $
 $ \Rightarrow {v = \dfrac{d}{t}} $
 $ \Rightarrow {v = \dfrac{{2d}}{t}} $
 $ \Rightarrow {v = \dfrac{{2 \times 350}}{2}} $
 $ \Rightarrow {v = 350} {\raise0.7ex\hbox{ $ m $ } \!\mathord{\left/
 {\vphantom {m s}}\right.}
\!\lower0.7ex\hbox{ $ s $ }} $ .

Note
Echo needs a hard surface of any object to bounce back the sound and soft surface to observe the sound waves.
Bat is also used in dark night as well as SONAR (Sound Navigation and Ranging) is also working based on this concept to detect the object in sea or mines. Sonar uses the Sound waves and ultra waves.
The sensation of hearing any sound persists in our brain for 0.1 second, this is called the ‘persistence of hearing’. Hence, the persistence of hearing in the human ear is 0.1 second.
Generally, an echo is used to analyze the speed of sound in a medium. Exploration of underwater petroleum deposits is done by detecting echo of shock waves produced by explosions on the water surface. The wave phenomena like total internal reflection, refraction and interference are also given by sound waves similar to those of the light waves.
Sound having three categories based on its frequency range that are – First is Infrasonic Sound, Second is Sonic Sound and third is Ultrasonic or supersonic sound.