Multiple choice

A man rows to a place 40 km distant and back in a total of 18 hours. He finds that he can row 5 km with the stream in the same time as 4 km against the stream. What is the speed of boat in still water? एक व्यक्ति 40 किमी की दूरी 18 घंटे में खे कर जाता है और वापस आ जाता है। वह पाता है कि जितने समय में वह धारा के अनुकूल 5 किमी खेता है उतने समय में धारा के प्रतिकूल 4 किमी खेता है। शांत जल में नाव की गति क्या है?

  1. 4

  2. 3

  3. 3.5

  4. 4.5

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D Correct answer
Explanation

Let boat speed in still water be b km/h and stream speed be s km/h. Downstream speed = (b + s) and upstream speed = (b - s). Given: 5/(b + s) = 4/(b - s), so 5(b - s) = 4(b + s), giving b = 9s. Total journey: 40/(b + s) + 40/(b - s) = 18 hours. Substituting b = 9s: 40/(10s) + 40/(8s) = 18, so 4/s + 5/s = 18, giving 9/s = 18, thus s = 0.5 km/h. Therefore b = 9 × 0.5 = 4.5 km/h.