A motorcyclist drives from A to B with a uniform speed of $30 k m h^{- 1}$ and returns with a speed of $20 k m h^{- 1}$ . Find the average speed.

24 k m h^{- 1}

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25 k m h^{- 1}

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12 k m h^{- 1}

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37 k m h^{- 1}

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Solution

The correct option is B $24 k m h^{- 1}$
Let the length of the line segment AB be $x m$

The time taken is given as t=distance covered/speed
Total time = $t_{1} + t_{2} = \frac{x}{30} + \frac{x}{20} = \frac{5 x}{60} h$
Average speed = Total distance/Total time = $\frac{2 x}{5 x / 60} = 24 k m / h$

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