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+1 vote
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in Distance, Time and Speed by (49.2k points)
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A streamer goes downstream from one port to another in 4 hours. It covers the same distance upstream in 5 hours. If the speed of the stream is 2 km/hr, the distance between the two ports is 

(a) 50 km 

(b) 60 km 

(c) 70 km 

(d) 80 km

1 Answer

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by (46.2k points)
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Best answer

(d) 80 km

Let the distance between the two parts be x km. 

Then, speed downstream =\(\frac{x}{4}\) km/hr 

and speed upstream = \(\frac{x}{5}\) km/hr 

⇒ Speed of stream = \(\frac12\big(\frac{x}4-\frac{x}5\big)\)

Given, \(\frac12\big(\frac{x}4-\frac{x}5\big)\) = 2 ⇒ \(\frac{x}4-\frac{x}5=4\)

⇒ \(\frac{x}{20}=4\) ⇒ x = 80 km.

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