Question

Snehal can row $28$ km downstream and $12$ km upstream in $5$ hours. He can row $21$ km downstream and $10$ km upstream in $4$ hours. Find the speed of Snehal in still water (in km/hr)

• A. $6$
• B. $9$
• C. $12$
• D. $15$

Answer 1

The correct option is B $9$

Let the speed of upstream be $x$ km/hr and speed of downstream be $y$ km/hr.

$\frac{12}{x}+\frac{28}{y}=5$ ....(1)

and $\frac{10}{x}+\frac{21}{y}=4$ ....(2)

Multiply equation (1) by $10$ and equation (2) by $12$, we get

$\frac{120}{x}+\frac{280}{y}=50$ ...(3)

and $\frac{120}{x}+\frac{252}{y}=48$ ....(4)

Subtract equations (3) and (4), we get

$\frac{28}{y}=2$

$\Rightarrow y=14$

Put this value in equation (1), we get

$\frac{12}{x}+\frac{28}{14}=5$

$\Rightarrow \frac{12}{x}=3$

$\Rightarrow x=4$

Thus, speed of snehal in still water $=\frac{1}{2}(14+4)=9$ km/hr.