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Speed of stream

CMAT 2019 · Quantitative Techniques & Data Interpretation · Medium · Time Speed Distance

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The speed of a boat in still water is 5 km/hr5\text{ km/hr}. If it takes thrice as much time in going 20 km20\text{ km} upstream as in going the same distance downstream, find the speed of the stream.

  1. A.

    5.5 km/hr

  2. B.

    2.5 km/hr

  3. C.

    3.5 km/hr

  4. D.

    4.5 km/hr

Answer

B

Explanation

Let stream speed be v km/hrv\text{ km/hr}. Upstream speed =5−v= 5 - v, Downstream speed =5+v= 5 + v.

Given: tup=3⋅tdownt_{\text{up}} = 3 \cdot t_{\text{down}} 205−v=3×205+v\frac{20}{5 - v} = 3 \times \frac{20}{5 + v} 5+v=3(5−v)  ⟹  5+v=15−3v  ⟹  4v=10  ⟹  v=2.5 km/hr5 + v = 3(5 - v) \implies 5 + v = 15 - 3v \implies 4v = 10 \implies v = 2.5\text{ km/hr}

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