A man rows a boat a certain distance downstream in 9 hours, while it takes 18 hours to row the same distance upstream. How many hours will it take him to row three-fifth of the same distance in still water?

Question: A man rows a boat a certain distance downstream in 9 hours, while it takes 18 hours to row the same distance upstream. How many hours will it take him to row three-fifth of the same distance in still water?

Solution:
Let distance = d
So downstream speed = d/9​
And upstream speed = d/18​

We know,
Speed in still water = (Downstream + Upstream)/2
= (d/9 + d/18)/2
= {(2d + d)/18}/2
= (d/6)/2
= d/12
So, still water speed = d/12
Distance to travel = 3d/5

∴ Time = (3d/5)/(d/12) = (3 × 12)/5 = 36/5 = 7.2 hours

So it will take the man 7.2 hours to row three-fifth of the distance in still water.