Multiple choice

A car travels first $\dfrac{1}{3}$ of the distance AB at $30km/h$,next$\dfrac{1}{3}$ of the distance at $40km/h$ and last $\dfrac{1}{3}$ of the distance at $24km/h$ Its average speed in km/h for the whole journey is

  1. $40$
  2. $35$
  3. $30$
  4. $28$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Average speed = Total distance / Total time. Let total distance = 3D. Time = D/30 + D/40 + D/24 = (4D + 3D + 5D) / 120 = 12D/120 = D/10. Average speed = 3D / (D/10) = 30 km/h.

AI explanation

Apply the harmonic mean formula for three equal distances: 3abc / (ab + bc + ca). Inserting the speeds 30, 40, and 24 into the equation gives (3 * 30 * 40 * 24) / (30*40 + 40*24 + 24*30). Solving the numerator (86400) and denominator (2880) results in an average speed of 30 km/h.

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