V = f x wavelength = 20000Hz x 0.075m = 1500 but V = $\frac{d}{t}$ = $\frac{2d}{3}$ (echo)$\rightarrow$ 2d = 4500 $\rightarrow$ d = 2250 m
johnmulu answered the question on February 11, 2017 at 08:44
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