Multiple choice

A boat can cover certain distance in downstream in 1 hr. and it takes 1½ hr. to cover same distance in upstream. If speed of the stream is 3 kmph, then what will be time taken by the boat to travel a distance of 75 km in still water? एक नाव अनुप्रवाह में निश्चित दूरी को 1 घंटे में तय करती है और यह उसी दूरी को ऊर्ध्वप्रवाह में तय करने के लिए 1½ घंटे लेती है | यदि धारा की चाल 3 किमी./घंटा है, तो शांत जल में नाव को 75 किमी. की दूरी तय करने में लिया गया समय क्या है?

  1. 5.5 hours/घंटे

  2. 4 hours/घंटे

  3. 3 hours/घंटे

  4. 5 hours/घंटे
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let boat speed in still water = b kmph, stream speed = 3 kmph. Downstream speed = b + 3, upstream speed = b - 3. If distance is D, then D/(b+3) = 1 hour, and D/(b-3) = 1.5 hours. So D = b + 3 and D = 1.5(b - 3). Equating: b + 3 = 1.5b - 4.5, so 0.5b = 7.5, b = 15 kmph. Time for 75 km in still water = 75/15 = 5 hours. The key is to find the boat's speed in still water first using the ratio of downstream and upstream times.

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