Question

A person can row at a speed of $4\frac{1}{2}$ km/hr in still water. If (s)he takes two times as long to row a distance upstream as to row the same distance downstream, then the speed of stream is

• A. $1$ km/hr
• B. $1.5$ km/hr
• C. $2$ km/hr
• D. $2.5$ km/hr

Answer 1

The correct option is B $1.5$ km/hr

Speed of man in still water $=\frac{9}{2}$ km/hr
Let speed of stream be $y$ km/hr.
Then, downstream speed $=\frac{9}{2}+y$
and upstream speed $=\frac{9}{2}-y$
According to question,
$2\times \frac{\text{Distance}}{\frac{9}{2}+y}=\frac{\text{Distance}}{\frac{9}{2}-y}$
$\Rightarrow \frac{2}{\frac{9}{2}+y}=\frac{1}{\frac{9}{2}-y}$
$\Rightarrow \frac{2}{9+2y}=\frac{1}{9-2y}$
$\Rightarrow 18-4y=9+2y$
$\therefore y=\frac{3}{2}=1.5$ km/hr