Discussion
The speed of a car is 90 km in the first hour and 60 km in the second hour. What is the average speed of the car?
Answer: B
Here the two times are equal - one hour each - not the distances, so the plain arithmetic mean is correct.
\[ \bar{v} = \frac{90 + 60}{1 + 1} = 75 \text{ kmph} \]
Had the two distances been equal instead, the harmonic mean \( \tfrac{2 \times 90 \times 60}{150} = 72 \) kmph would apply - which is exactly the first distractor.
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