Correct Answer - D

According to question ,

Downstream speed = `(18)/(3)` = 6 km/hr

or x + y = 6 (when `x to` speed of boat in still water ,

`y to` speed of current )

Speed of current = `(1)/(3) xx 6 = 2` km/hr

Speed of boat in still water = 4 km/hr

Required time = `(50)/((4-2)) = 25` hour