A boat takes two hours to travel 8 km and

Boat covers distance of $16\,km$ in a still water in 2 hours.

i.e. ${v_B} = \frac{{16}}{2} = 8\,km/hr$

Now velocity of water $ \Rightarrow {v_w} = 4km/hr$.

Time taken for going upstream

${t_1} = \frac{8}{{{v_B} - {v_w}}}$$ = \frac{8}{{8 - 4}} = 2hr$ (As water current oppose the motion of boat)

Time taken for going down stream

${t_2} = \frac{8}{{{v_B} + {v_w}}}$ $ = \frac{8}{{8 + 4}} = \frac{8}{{12}}hr$ (As water current helps the motion of boat)

$\therefore $ Total time $ = {t_1} + {t_2} = \left( {2 + \frac{8}{{12}}} \right)hr$ or $2\,hr\, 40\,min$