Multiple choice

If a boat goes a certain distance at 30 km/h and comes back the same distance at 20 km/hr, what is the average speed (in km/hr) for the total journey?

  1. 25

  2. 24.5

  3. 24

  4. 25.5
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Average speed is calculated as total distance divided by total time. If the distance is d, the time taken for the first leg is d/30 and the return leg is d/20. Total time is d/30 + d/20 = 5d/60 = d/12. Average speed = 2d / (d/12) = 24 km/hr.

AI explanation

When a boat covers equal distances in two legs of a journey at different speeds, we use the harmonic mean formula for average speed, which is 2XY divided by (X plus Y). Substituting the downstream speed of 30 km/hr and upstream speed of 20 km/hr gives 2 multiplied by 30 multiplied by 20 divided by 50. This calculates to 1200 divided by 50, resulting in an average speed of 24 km/hr.

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