The time taken by sound to travel from observer to cliff and then from cliff to the man on sea shore $=3$ seconds.
Let the distance between the cliff and the observer on the ship = ' $\mathit{x}$ ' metres.
$\therefore$ Distance travelled by sound
$=\left(\mathit{x}+\mathit{x}+640\right)\mathrm{m}$
$=\left(2\mathit{x}+640\right)\mathrm{m}$



$\therefore \left(2\mathit{x}+640\right)=320\times 3$
$2\mathit{x}+640=960$
$2\mathit{x}=960-640$
$2\mathit{x}=320$
$\mathit{x}=\frac{320}{2}=160\mathrm{m}$