Multiple choice

A motor boat can travel at 8 km/hour in still water. It travelled 60 km downstream in a river and then returned to the same place in 20 hours. Find the rate of flow of river. एक मोटर-बोट शांत जल में 8 किमी/घं. की गति से चल सकती है। वह नदी में 60 किमी. अनुप्रवाह चली और वही लौट आयी। उसे कुल 20 घंटे लगे। नदी के प्रवाह की गति क्या है?

  1. 2 km/h/किमी/घं.

  2. 3 km/h/किमी/घं.

  3. 4 km/h/किमी/घं.

  4. 3.5 km/h/किमी/घं.
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C Correct answer
Explanation

Let the river flow at v km/h. Downstream speed = 8+v km/h, upstream speed = 8-v km/h. Time downstream = 60/(8+v), time upstream = 60/(8-v). Total time: 60/(8+v) + 60/(8-v) = 20. Multiplying by (8+v)(8-v): 60(8-v) + 60(8+v) = 20(64-v²). This simplifies to 960 = 1280 - 20v², so 20v² = 320 and v² = 16. Therefore v = 4 km/h.

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