Multiple choice

The speed of current is 5 km./hr. A motorboat goes 10 km. upstream and back again to the starting point in 50 minutes. The speed in km./hr. of the motor boat in still water is - एक धारा की चाल 5 किमी./घं. है। एक मोटरवोट धारा के विपरीत 10 किमी. जाने में और वापस आरम्भिक बिन्दु पर आने में 50 मिनट लेती है। तो स्थिर पानी में उस मोटर बोट की गति कितने किमी./घं. होगी?

  1. 20

  2. 26

  3. 25

  4. 28
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let boat speed in still water = x km/hr. Speed upstream = x - 5, speed downstream = x + 5. Time upstream + time downstream = 50/60 hr. So 10/(x-5) + 10/(x+5) = 50/60 = 5/6. Multiplying through: 60(x+5) + 60(x-5) = 5(x²-25). This gives 5x² - 120x - 175 = 0, or x² - 24x - 35 = 0. Factoring: (x - 25)(x + 1) = 0, so x = 25 km/hr.

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