Question

A boat takes $2$ hours to travel $8km$ and back in still water lake with water velocity of $4km/h$, then the time taken for going upstream of $8km$ and coming back is

• A. 120 min
• B. 160 min
• C. 200 min
• D. None of these

Answer 1

Answer: (B)

160 min

The velocity of boat in still water is $v_b=\frac{8+8}{2}=8km/h$

Now when velocity of water is given as $v_r=4km/h$

Time taken for downstream is $t_d=\frac{8}{v_b+v_r}=\frac{8}{12}=\frac{2}{3}h$

Time taken for upstream is $t_u=\frac{8}{v_b-v_r}=\frac{8}{4}=2h$

So total time taken $t=t_d+t_u=2+\frac{2}{3}=\frac{8}{3}h$

$⟹$ $t=60\times \frac{8}{3}min=160min$