Question

If a person rowing at the rate of $5$ km/h in still water, takes thrice as much time in going $40km$ upstream as in going $40km$ downstream, then the speed of the stream, is:

• A. $7.5km/hr$
• B. $2.5km/hr$
• C. $3.5km/hr$
• D. $1.5km/hr$

Answer 1

The correct option is B $2.5km/hr$

Consider,

Speed of rowing by a person $=5$ km/hr.

Speed of stream $=x$ km/hr
Upstream Speed $=(5-x)$ km/hr
Downstream speed $=(5+x)$ km/hr
According to the question,

$\Rightarrow \frac{40}{5-x}=\frac{3\times 40}{5+x}$

$\Rightarrow 5+x=15-3x$

$\Rightarrow 4x=10$

$\Rightarrow x=2.5$ km/hr