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A man drives from town A to town B at an average speed of 63 km/h and comes back along the same route at 42 km/h. What is his average speed for the entire round trip?

1. 52.5 km/h
2. 55.4 km/h
3. 48.5 km/h
4. 50.4 km/h

Correct answer: 50.4 km/h

Solution

For equal distances, average speed = 2ab/(a+b) = 2(63)(42)/(63+42) = 5292/105 = 50.4 km/h. The arithmetic mean (52.5) would be wrong because more time is spent at the slower speed.

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