If the speed of the motor boat in still water is 24 km/hr. It takes thrice as long as to go downstream to a place as to return downstream to the starting place, then speed of the stream is _____________.

12 km/hr

The correct answer is Option (2) → 12 km/hr

Let the speed of the motorboat in still water be 24 km/hr, and speed of stream be x km/hr.

Downstream speed = $(24+x)$ km/hr

Upstream speed = $(24-x)$ km/hr

$\frac{\text{Distance}}{\text{Upstream Speed}}=3×\frac{\text{Distance}}{\text{Downstream Speed}}$

$\frac{1}{24-x}=3×\frac{1}{24+x}$

$⇒4x=48$

$⇒x=12$ km/hr