Discussion
A motorist travel to a place 150 km away at an average speed of 50 km/hr and returns ar 30 km/hr.His average speed for the whole journey in km/hr is:
Answer: B
Equal distances each way call for the harmonic mean.
\[ \bar{v} = \frac{2 \times 50 \times 30}{50 + 30} = \frac{3000}{80} = 37.5 \text{ km/hr} \]
The 150 km is not needed - it cancels. The arithmetic mean of 40 would be too high, since more time is spent on the slower return.
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