Multiple choice

A boat covers a distance of 30 km downstream in 2 hours while it takes 6 hours to cover the same distance upstream. If the speed of the stream is half that of the boat, then what is the speed of the boat in still water in km per hour? एक नाव धारा के अनुकूल 30 किमी की दूरी 2 घंटे में तय करती है जबकि समान दूरी को धारा के प्रतिकूल तय करने में 6 घंटे का समय लेती है। यदि धारा की गति नाव की गति से आधी है, तो शांत जल में नाव की गति किमी प्रति घंटा में क्या है?

  1. 10 km/hr 10 किमी / घंटा

  2. 8 km/hr 8 किमी / घंटा

  3. 12 km/hr 12 किमी / घंटा

  4. 16 km/hr 16 किमी / घंटा

  5. 18 km/hr 18 किमी / घंटा
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A Correct answer
Explanation

Downstream speed = 30/2 = 15 km/hr. Upstream speed = 30/6 = 5 km/hr. Let boat speed in still water = b, stream speed = s. We have: b + s = 15, b - s = 5. Also given s = b/2. Substituting: b + b/2 = 15, so 3b/2 = 15, b = 10 km/hr. Check: s = 5, b + s = 15 (downstream), b - s = 5 (upstream). Correct.

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