Multiple choice

The speed of current is 5 km/hr. A boat goes 10 km upstream and back to the starting point in 40 min. Find the speed of boat in still water.

  1. 15

  2. 25

  3. 22

  4. 35

  5. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let boat speed = b km/hr. Speed upstream = b - 5, downstream = b + 5. Time = distance/speed, so 10/(b-5) + 10/(b+5) = 40/60 = 2/3 hr. Multiplying by 3(b-5)(b+5): 30(b+5) + 30(b-5) = 2(b² - 25). This gives 60b = 2b² - 50, or b² - 30b - 25 = 0. Solving: b = 25 km/hr (ignoring negative root). Option B is correct.

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