A motor boat goes 24 km downstream and comes back to the starting point in 6 hours. If the speed of the boat in still water is 9 km/hr, then the speed of the stream is :

3 km/hr

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

Let the speed of the stream be x km/hr

Downstream speed = $9+x$

Upstream speed = $9-x$

$⇒\frac{24}{9+x}+\frac{24}{9-x}=6$  $[Time=\frac{Distance}{speed}]$

$⇒24(9-x)+24(9+x)=6(9+x)(9-x)$

$⇒432=486-6x^2$

$⇒x^2=9⇒x=3km/hr$