A car travels from city A to city B at 45 km/h and returns along the same road at 90 km/h. What is its average speed for the whole round trip?
Answer and explanation
Correct answer: B. 60
- Step 1: For a one-way distance d, the total time is d/45 + d/90 and the total distance is 2d.
- Step 2: Average speed = 2d / (d/45 + d/90) = 60 km/h.
Answer
60.
Why not the others: 67.5 is the simple mean of the two speeds, but more time is spent at the lower speed; the sum and the difference of the speeds are not averages.
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