Multiple choice

A car completes the journey from $A$ to $B$ at a constant speed of $20 {km}/{hr}$ and covers the return journey from $B$ to $A$ at a constant speed of $30 {km}/{hr}$. The average speed for the total journey from $A$ to $B$ and back is

  1. $24 {km}/{hr}$
  2. $\sqrt{600} {km}/{hr}$
  3. $25 {km}/{hr}$
  4. $50 {km}/{hr}$
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A Correct answer
Explanation

Average speed = 2 * v1 * v2 / (v1 + v2) = 2 * 20 * 30 / (20 + 30) = 1200 / 50 = 24 km/hr.

AI explanation

Using the harmonic mean formula for equal distances travelled at two different speeds, the average speed is 2xy / (x + y). Substituting the given speeds of 20 km/hr and 30 km/hr gives (2 * 20 * 30) / (20 + 30). This simplifies to 1200 / 50, which equals 24 km/hr.

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