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Arun's speed of swimming in still water is 5 km/hr5 \mathrm{~km} / \mathrm{hr}. He swims between two points in a river and returns back to the same starting point. He took 2020 minutes more to cover the distance upstream than downstream. If the speed of the stream is 2 km/hr2 \mathrm{~km} / \mathrm{hr}, then the distance between the two points is :

Solution

✅ Correct Option: 3

We are given:

  • Arun's speed in still water = 5 km/hr
    • Speed of stream = 2 km/hr
    • Time difference between upstream and downstream = 20 minutes = 13\frac{1}{3} hour

Let the distance between the two points be DD km (one-way).

  • Downstream speed =5+2=7= 5 + 2 = 7 km/hr
    • Upstream speed =5−2=3=5 - 2 = 3 km/hr

Time taken downstream = D7\frac{D}{7} hours

Time taken upstream = D3\frac{D}{3} hours

We’re told upstream takes 20 minutes (i.e., 13\frac{1}{3} hr) more than downstream:

D3−D7=13\dfrac{D}{3} - \dfrac{D}{7} = \dfrac{1}{3}

⇒D(13−17)=13\Rightarrow D\left(\frac{1}{3} - \frac{1}{7}\right) = \frac{1}{3}

⇒D(7−321)=13\Rightarrow D\left(\frac{7 - 3}{21}\right) = \frac{1}{3}

⇒D⋅421=13\Rightarrow D \cdot \frac{4}{21} = \frac{1}{3}

⇒D=13⋅214=74=1.75\Rightarrow D = \frac{1}{3} \cdot \frac{21}{4} = \frac{7}{4} = 1.75 hours.

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