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?

7.2

According to the question,

Let's assume the total distance = 180 km

So, down-stream speed will be = 180/9 = 20 km/hr

So, up-stream speed will be = 180/18 = 10 km/hr

Now, speed of the boat will be = (20 + 10)/2 = 15 km/hr

So,the boat can row (3/5^th of 180km) 108 km in 108/15 = 7.2 hr