Multiple choice

In a stream running at 5 kmph a motor goes 15 km upstream and back again to the starting point in 2 h 15 minutes. Find the speed of the motor boat in still water.

  1. 10 kmph

  2. 15 kmph

  3. 12 kmph

  4. 18 kmph

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

Let boat speed in still water be x kmph. Upstream speed = x-5, downstream speed = x+5. Time upstream = 15/(x-5), time downstream = 15/(x+5). Total time: 15/(x-5) + 15/(x+5) = 2.25 hours. Solving gives x=15 kmph. This matches Option B. Key insight: boat speed relative to stream changes direction, but distance remains constant.

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