Aptitude 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**

**Chirag Goyal**

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Speed in Downstream is 18km/hr.

Speed in Upstream is 10km/hr.

**Jeetu**

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

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 ${\dfrac{18-10}{2}} = 4 km/hr.$