Multiple choice

A man hoes from a place A to B at a speed of 12km/hr and returns from B to A at a speed of 18 km/hr. The average speed for the whole journey of distance 12km is :

  1. $14\dfrac{2}{5}km/hr$
  2. $15\,km/hr$
  3. $15\dfrac{1}{2}km/hr$
  4. $16\,km/hr$
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A Correct answer
Explanation

For equal distances, average speed is the harmonic mean of the two speeds. Thus, average speed = 2 × 12 × 18/(12 + 18) = 14.4 km/hr, which is 14 2/5 km/hr.

AI explanation

For a journey where equal distances are travelled at two different speeds, the average speed is calculated using the formula 2xy / (x + y). Substituting the speeds 12 km/hr and 18 km/hr gives (2 * 12 * 18) / (12 + 18). This simplifies to 432 / 30, which equals 14.4 or 14 2/5 km/hr.