Multiple choice

Speed of a boat is 8 km/hr in still water and the speed of the stream is 2 km/hr. If the boat takes 8 hours to go to a place and come back, then what is the distance (in km) of the place? एक नाव की स्थिर जल में चाल 8 किमी / घंटा है और धारा की चाल 2 किमी / घंटा है। यदि नाव को किसी स्थान पर जाने और वापस आने में 8 घंटे लगते हैं, तो उस स्थान की दूरी (किमी में) क्या है?

  1. 24

  2. 30

  3. 45

  4. 42
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B Correct answer
Explanation

For a round trip, the boat's downstream speed is 8+2=10 km/hr and upstream speed is 8-2=6 km/hr. Let the distance be d. Total time is d/10 + d/6 = 8 hours. Solving: (3d+5d)/30 = 8, so 8d = 240 and d = 30 km. The distance is the same in both directions, so we use the harmonic mean relationship.

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