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?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 7.2.
Let the distance, speed of the boat in still water, and speed of the river be D, S, and R respectively.
According to the concept,
D/(S - R)=18 ...(1)
D/(S + R) = 9 ...(2)
Now, dividing equation (1) and (2),
(S + R)/(S - R) = 2
⇒ (S + R + S - R)/(S + R - S + R) = (2 + 1)/(2 - 1) (Componendo-Dividendo Method)
⇒ S/R = 3
⇒ S = 3R
Putting S = 3R in (1),
⇒ D = 36R
Now, time taken to row three-fifth of the same distance in still water
= (36R × 3/5) ÷ 3R
= 7.2 hours
Hence, It will take 7.2 hours to row three-fifth of the same distance in still water.