A man rows a boat downstream in 3 hrs and returns back in 6 hours. If the speed of boat in still water is 6km/hr, then speed of stream is (in km/hr) :

2

The correct answer is Option (3) → 2

Let speed of the stream be $x\,km/hr$ and distance traveled downstream and upstream be D km.

Downstream speed = $6+x\,km/hr$

Upstream speed = $6-x\,km/hr$

$⇒3(6+x)=6(6-x)$  $\left[2×\frac{D}{6+x}=\frac{D}{6-x}\right]$

$⇒18+3x=36-6x$

$⇒9x=18$

$⇒x=2\,km/hr$