Question

A man rows downstream $30$ km and upstream $18$ km, taking $5$ hours each time. The velocity of current is ____________.

• A. $1.2$ km/hr
• B. $6$ km/hr
• C. $3.6$ km/hr
• D. $2.4$ km/hr

Answer 1

The correct option is A $1.2$ km/hr

Let the speed of man is $u$ and that of current is $v$

Downstream speed will be $u+v$

Upstream speed will be $u-v$

He travels $30$ km downstream in $5$ hrs

So, downstream speed is $\frac{30}{5}=6$ km/hr

Similarly upstream speed will be $\frac{18}{5}=3.6$ km/hr

So, $u+v=6$

$\Rightarrow u-v=3.6$
Subtracting both the equations;

$\Rightarrow 2v=2.4$

$\Rightarrow v=1.2$ km/hr