Discussion
A man covers half of the journey at the speed of 70 km/h and the remaining half at 30 km/h. What is his average speed for the whole journey?
Answer: C
The average speed is 42 km/h. For equal distances the average speed is the harmonic mean, not the arithmetic mean: \[ \frac{2 \times 70 \times 30}{70 + 30} \] \[ = \frac{4200}{100} = 42 \text{ km/h} \]
50 km/h is the arithmetic mean of 70 and 30, which would be right only for equal times, and 45 and 55 km/h do not follow from either rule.
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