Question

A man can row a boat with $4km/h$ in still water. If he is crossing a river where the current is $2km/h$ and width of river is $4km$. How long will it take him to row $2km$ up the stream and then back to his starting point?

• A. $2hrs$
  
• B. $1hr$
  
• C. $\frac{4}{3}hrs$
  
• D. None of the above
  

Answer 1

The correct option is C $\frac{4}{3}hrs$

Given :

Velocity of man wrt water, $V_{MW}=4$ km/hr

Velocity of river, $V_R=2$ km/hr

Distance covered in upstream or downward motion $d=2$ km each

For upstream motion of the boat :

Velocity of the man , $V_M=V_{MW}-V_W=4-2=2$ km/h

$\therefore$ Time taken to go upstream $t_1=\frac{2}{2}=1$ hr

For downward motion of the boat or (return journey) :

Velocity of the man $V_M=V_{MW}+V_W=4+2=6$ km/h

$\therefore$ Time taken to go downstream $t_2=\frac{2}{6}=\frac{1}{3}$ hr

$\therefore$ Total time taken $T=t_1+t_2=1+\frac{1}{3}=\frac{4}{3}$ hrs