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