A motor boat goes 20 km downstream and comes back to the starting point in 6 hours. If the speed of the boat in still water is 12 km/h, then the speed of the stream is :

8 km/h

The correct answer is Option (3) → 8 km/h

Speed Downstream (boat + stream) = (12 + x) km/hr

Speed Upstream (boat - stream) = (12 - x) km/hr

$⇒\frac{20}{12+x}+\frac{20}{12-x} =6$

$⇒\frac{240×2}{144-x^2}=6$

$⇒480=864-6x^2$

$⇒6x^2=384$

$⇒x=8\,km/hr$