A boat takes two hours to travel 8 km and back in still water. If the velocity of water 4 kmh^–1, the same taken for going upstream 8 km and coming back is ………minutes.

Boat covers distance of 16 km in a still water in 2 hours:

i.e., $v_B=\frac{16}{2}=8 {\mathrm{kmh}}^{-1}$

Now, velocity of water

$v_w=4 {\mathrm{kmh}}^{-1}$

As water current opposes the motion of boat, time taken for going upstream

$t_1=\frac{8}{v_B-v_w}=\frac{8}{8-4}=2 \mathrm{h}=120 \min$

As water current supports the motion of boat, time taken for going downstream

$t_2=\frac{8}{v_B+v_w}=\frac{8}{8+4}=\frac{8}{12} \mathrm{h}=40 \min$

$\therefore \mathrm{Total} \mathrm{time}=\mathrm{t}={\mathrm{t}}_1+{\mathrm{t}}_2=160 \min$