A person rings a metallic bell near a strong concrete wall. He hears the echo after 0.3 s. If the sound moves with a speed of 340 m/s, how far is the wall from him?

  1. 102 m
  2. 11 m
  3. 51 m
  4. 30 m


An echo is the sound you hear when you make a noise and the sound wave reflects off a distant object. The sound wave travels in a straight line. When the wave hits a solid wall, it is reflected.

d = v × t = 340 m/s × 0.3 s

d = 102 m

d is the distance the sound wave traveled back and forth. The distance of the wall from the person will be half of this.

Required distance = d/2 = 102/2 = 51 m

The correct option is C.

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