Question

A man can row $40$km upstream and $30$km downstream is $8$ hours and $30$ km upstream and $50$km downstream in $\frac{35}{4}$ hours. The speed of the stream is?

• A. $10$ km/hr
• B. $1$ km/hr
• C. $5$ km/hr
• D. $2$ km/hr

Answer 1

The correct option is A $10$ km/hr

$\text{Solve: Let upstream speed}=xkm/hour$

$\text{and downstream speed}=y\text{km/hour}$

$\text{we know that}$

$\text{time}=\frac{\text{Tolal distance}}{\text{speed}}$

$\text{according to given condition}$

$8=\frac{40}{x}+\frac{30}{y}-\left(i\right)$

and

$\frac{35}{4}=\frac{30}{x}+\frac{50}{y}-\left(ii\right)$

from equation (i) and (ii) we get,

$x=8$ and $y=10$

during downstream flow of water support man so, speed (man + seream)

increases. during upstream speed (man-stream) descreas

$\Rightarrow$Speed of man + speed of stream $=10$

and speed of man - speed of stream $=8$

$\Rightarrow$ Speed of stream is 1 km/hr and Speed of man in still water is a 9 km/hr