Easy Time, Speed & Distance Solved QuestionAptitude Discussion

 Q. A man can row 4.5 km/hr in still water and he finds that it takes him twice as long to row up as to row down the river. Find the rate of the stream.
 ✖ A. 2 km/hr ✖ B. 2.5 km/hr ✔ C. 1.5 km/hr ✖ D. 1.75 km/hr

Solution:
Option(C) is correct

Let the speed of the current be $x$ km/hr

Thus upward speed = $(4.5-x)$ km/hr

and downward speed = $(4.5+x)$ km/hr

Let distance travelled be $y$, then for the same distance $y$,

$\text{Time Rowing Upwards}$$=2\times \text{Time Rowing Downwards}$

$\dfrac{y}{4.5-x}=2 \times \dfrac{y}{4.5+x}$

⇒ $x=$ 1.5 km/hr

Edit: Thank you Sid for pointing out the typo, upwards and downwards speeds have been corrected.

(3) Comment(s)

()

Let man’s upstream rate be x kmph.

Then, his downstream rate= 2x kmph

Rate in still water = 1/2 (2x + x) kmph = 1.5x kmph

So, 1.5x = 4.5 or x = 3

Upstream Rate= 3 km/hr, Downstream Rate= 6 km/hr

Hence, rate of stream = 1/2 (6 – 3) km/hr = 1.5 km/hr

Sid
()

A correction. Speed downstream is $4.5+x$ and the speed upstream will be $4.5-x$ :)

Deepak
()

Thank you Sid for pointing out the typo, I've modified and corrected the solution.