Correct Answer - C
Let the speed of stream be x km/hr
Then ,
Speed of boat in upstream = ` x xx (400)/(100) = 9`
`implies x = 2.25` km/hr
Speed of boat in still water = `9 + 2.25 = 11.25` km/hr
Distance covered in 3 hours in downstream
= `(11.25 + 2.25) xx 5 = 67.5` km