Multiple choice

The speed of a boat in still water is 9 km/h and the speed of the stream is 3 km/h. The boat rows to a place which is 47 km away and returns from the same path. Find the average speed of the boat during the whole journey.

  1. 6 km/h

  2. 12 km/h

  3. 5 km/h

  4. 8 km/h

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

The downstream speed of the boat is 9 + 3 = 12 km/h, and the upstream speed is 9 - 3 = 6 km/h. Since the distance traveled in both directions is equal, the average speed is given by the harmonic mean of the two speeds, which is 2 * 12 * 6 / (12 + 6) = 8 km/h.

AI explanation

The downstream speed is 9 + 3 = 12 km/h and the upstream speed is 9 - 3 = 6 km/h. For the same distance of 47 km, the average speed for the whole journey is found by the harmonic mean formula, which is 2 * 12 * 6 / (12 + 6). This calculates to 144 / 18 = 8 km/h. The result is 8 km/h.

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