Time, Speed & Distance
Aptitude

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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

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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'