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A person travels by car from city A to city B at an average speed of 63 km/h and returns at an average speed of 42 km/h. His average speed for the whole journey is:

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

Since the person travels the same distance each way, the average speed is not the arithmetic mean. Use the formula \(\frac{2ab}{a+b}\) with speeds 63 and 42. This gives \(\frac{2 \times 63 \times 42}{63+42} = 50.4\) km/h.

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