The sum of speed of a boat in still water and speed of stream is 18 km/hr and the boat take total 10 hours to cover 72 km distance in both downstream and upstream. Find speed of stream is what percent less than the upstream speed of the boat.

Correct option is B

Let speed of stream = x km/hr
So, speed of boat in still water = (18-x) km/hr
ATQ, 72/18 + 72/(18-2x) = 10
72/(18-2x) = 6
18 – 2x = 12
x = 3
speed of boat in still water = 15 km/hr
Upstream speed of boat = 15 – 3 = 12 km/hr
Required percent = (12-3)/12×100=75%