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Joel covers the distance from A to B at a speed of 18 km/h and returns from B to A by car at a speed of 36 km/h. What is his average speed for the entire journey?

1. 18 km/h
2. 30 km/h
3. 24 km/h
4. 36 km/h

Correct answer: 24 km/h

Solution

Since the onward and return distances are equal, average speed is given by the harmonic mean of the two speeds. Using the formula, average speed = \(\frac{2ab}{a+b}\) = \(\frac{2\cdot18\cdot36}{18+36}=24\) km/h.

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