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A man travels the first half of a journey at $30$ km/hr and the second half at $60$ km/hr. Find his average speed for the whole journey.
A$35$ km/hr
B$40$ km/hr
C$45$ km/hr
D$50$ km/hr
Answer & Solution
Correct answer: B. $40$ km/hr
For equal distances, average speed is the harmonic mean: $\dfrac{2xy}{x+y} = \dfrac{2 \times 30 \times 60}{90} = \dfrac{3600}{90} = 40$ km/hr.
Not the arithmetic mean $(30 + 60)/2 = 45$ — that holds for equal *times*, not equal *distances*.
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