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