Question

An echo repeats four syllables. The distance of the reflecting surface, if it takes one-fifth of a second to pronounce or hear one syllable distinctly is (Velocity of sound $= 340\,ms^{-1}$)

• A. $100\,m$
• B. $136\,m$
• C. $120\,m$
• D. $50\,m$

Answer 1

Answer: (B)

$136\,m$

Answer 2

An 'echo repeats four syllables' means that if a man pronounces $a, b, c, d$ and $e$, he will hear the last four syllables, that is, he will hear $b, c, d, e$. It means that the sound wave of each syllable returns in $4 \times \frac{1}{5}\,s$ $\therefore 2D = 340 \times \frac{4}{5}$ or $D = 136 \,m$ where $D$ is the distance of reflecting surface