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

i.e. \(\mathbf{v_B} = \frac{16}{2} = 8 \mathrm{km/hr}\)

Now velocity of water \(\Rightarrow \mathbf{v}_w = 4 \text{ km/hr}\) .

Time taken for going upstream

\(t_{1} = \frac{8}{v_{B} - v_{W}} = \frac{8}{8 - 4} = 2 \mathrm{hr}\)

(As water current oppose the motion of boat)

Time taken for going down stream

\(t_2 = \frac{8}{v_B + v_w} = \frac{8}{8 + 4} = \frac{8}{12} \, \mathrm{hr}\)

(As water current helps the motion of boat)

Total time \(= t_{1} + t_{2} = \left( 2 + \frac{8}{12} \right) \mathrm{hr}\) or 2hr 40min