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 = ' $x$ ' metres.$\therefore$ Distance travelled by sound$\;=(x+x+640)\mathrm{m}$$\;=(2x+640)\mathrm{m}$$\;\text{ Speed }\;\;=320\;\mathrm{m} / \mathrm{s}$$\;\text{ time }\;\;=3\;\text{ seconds }\;$$\because\;\; \;\text{ Distance }\;\;=\;\text{ Speed }\; \times \;\text{ time }\;$$\therefore \;\; (2x+640)\;=320 \times 3$$2x+640\;=960$$2x\;=960-640$$2x\;=320$$x\;=\;\frac{320}{2}=160 \mathrm{m}$