Answer:
t=1.623 sec
Explanation:
The distance traveled before the echo is had is:
[tex]distance=2d, d=280\ m\\\\=280\times 2\\\\=560 \ m[/tex]
Given the speed of sound as v=345m/s, we use the speed equation to solve for t:
[tex]v=\frac{d}{t}\\\\345\ m/s=\frac{560m}{t}\\\\t=\frac{560}{360}\\\\=1.623 \ s[/tex]
Hence, it takes 1.623 seconds to hear the echo.