Multiple choice

A boy walks to his school at a distance of 6 km with constant speed of $ 2.5 kmg^{-1}$ and walks back with a constant speed of $4kmh^{-1}$. His average speed for round trip expressed in $kmh^{-1}$, is

  1. $\dfrac{24}{13}$
  2. $\dfrac{40}{13}$
  3. $3$
  4. $\dfrac{1}{2}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The total distance is 12 km. The total time is 6/2.5 + 6/4 = 3.9 hours, so average speed is 12/3.9 = 40/13 km/hr.

AI explanation

For a round trip where equal distances are traveled at different speeds, use the harmonic mean formula 2xy / (x + y). Substituting the given speeds of 2.5 km/h and 4 km/h gives (2 * 2.5 * 4) / (2.5 + 4), which simplifies to 20 / 6.5. The average speed for the round trip is 40/13 km/h.

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