The minimum distance of reflector surface from the source for listening the echo of sound is

(1) 28 m

(2) 18 m

(3) 19 m

(4) 16.5 m

(4) $2d=v×t$, where v = velocity of sound = 332 m/s

t = Persistence of hearing = $\frac{1}{10}\mathrm{sec}$.

$d=\frac{v×t}{2}=\frac{332×\frac{1}{10}}{2}=16.5m$

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