A man can row a boat in still water at the rate of 12 km/hr. He finds that it takes him thrice as much times to row upstream than downstream. The speed of the current is :

6 km/hr

The correct answer is Option (3) → 6 km/hr

Speed of man in still water = $12\, km/hr$

Speed of current = $x\, km/hr$

Downstream speed = $12+x\,km/hr$

Upstream speed = $12-x\,km/hr$

and,

$\frac{d}{12-x}=3×\frac{d}{12+x}$  [d = Distance]

$⇒12+x=36-3x$

$⇒4x=24$

$⇒x=6\,km/hr$