Question

A man can row a boat with $4km/hr$ in still water. If he is crossing a river where the current is $2km/hr$, how long will it take him to row $2km$ upstream and then back to his starting point?

• A. $\frac{3}{4}hr$
• B. $\frac{3}{2}hr$
• C. $\frac{2}{3}hr$
• D. $\frac{4}{3}hr$

Answer 1

The correct option is D $\frac{4}{3}hr$

Time taken to row $2km$ upstream, $T_1=\frac{Distance}{V_b-V_r}=\frac{2}{4-2}=1hr$
Time taken to row $2km$ downstream $T_2=\frac{Distance}{V_b+V_r}=\frac{2}{4+2}=\frac{1}{3}hr$
$\therefore \text{Total time}T=T_1+T_2=(1+\frac{1}{3})hr=\frac{4}{3}hr$