Multiple choice

A boat covered a certain distance upstream and returned to the starting point. Speed of the boat in still water was 10 kmph and speed of the stream was 5 kmph. What was the average speed of the boat for the whole journey?

  1. 7.5 kmph

  2. 6 kmph

  3. 8.5 kmph

  4. 10 kmph

  5. None of these
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A Correct answer
Explanation

Upstream speed = 10 - 5 = 5 kmph. Downstream speed = 10 + 5 = 15 kmph. Average speed for a round trip = (2 * upstream * downstream) / (upstream + downstream) = (2 * 5 * 15) / (5 + 15) = 150 / 20 = 7.5 kmph.

AI explanation

Using the boat and stream formula, the downstream speed is 10 + 5 = 15 kmph and the upstream speed is 10 - 5 = 5 kmph. The average speed for the whole journey is found by the formula 2 * S1 * S2 / (S1 + S2). Substituting the speeds gives (2 * 15 * 5) / (15 + 5) = 150 / 20 = 7.5 kmph. The average speed of the boat was 7.5 kmph.

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