A motor boat covers 36 km downstream and 24 km upstream. If the boat takes 6 hours to cover each distance, then the speed of the motor boat in still water is,

5 km/hr

The correct answer is Option (4) → 5 km/hr

let the speed of the boat be $x$ km/hr

the speed of the stream be $y$ km/hr

Downstream speed = $\frac{36}{x+y}=6$

Upstream speed = $\frac{36}{x-y}=6$

$⇒x+y=6$  ...(1)

$x-y=4$   ...(2)

Add (1) and (2)

$2x=10$

$x=5\, km/hr$