Moderate Time, Speed & Distance Solved QuestionAptitude Discussion

 Q. A man can row 50 km upstream and 72 km downstream in 9 hours. He can also row 70 km upstream and 90 km downstream in 12 hours. Find the rate of current.
 ✖ A. 3 km/hr ✖ B. 8 km/hr ✖ C. 4 km/hr ✔ D. None of these

Solution:
Option(D) is correct

Let $x$ and $y$ be the upstream and downstream speed respectively.

Hence

$\dfrac{50}{x}+\dfrac{72}{y}=9$ and

$\dfrac{70}{x}+\dfrac{90}{y}=12$

Solving for $x$ and $y$ we get $x=10$ km/hr and $y=18$ km/hr

We know that:

$\text{Speed of the stream}$

$=\dfrac{1}{2}\times \text{(downstream speed - upstream speed)}$

$=\dfrac{1}{2}(18-10)$

4 km/hr

(3) Comment(s)

Chirag Goyal
()

Wrong Option Marked Please update answer choice.

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Another way of solving such type of Questions

Subtract 1st situation from 2nd we've got 20 km upstream and 18 km downstream covered in 3 hrs.(Total)

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Using Hit and Trial Method

Let 20 km covered in 2 hrs and 18 km covered in 1 hr.

So Speed in Downstream is 10km/hr.

& Speed in Upstream is 18km/hr.

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and Rate of current is ${\dfrac{18-10}{2}} = 4 km/hr.$

Chirag Goyal
()

Speed in Downstream is 18km/hr.

Speed in Upstream is 10km/hr.

Jeetu
()

You are saying 4kmph is the answer but then why the answer is marked as 'None of these'