Discussion
A trained covered x km at 40 kmph and another 2x km at 20 kmph. Find the average speed of the train in covering the entire 3x km.
Answer: C
Add the times, not the speeds.
\[ t = \frac{x}{40} + \frac{2x}{20} = \frac{x}{40} + \frac{4x}{40} = \frac{5x}{40} = \frac{x}{8} \]
\[ \bar{v} = \frac{3x}{x/8} = 24 \text{ kmph} \]
Averaging the two speeds gives 30 kmph, too high - two thirds of the journey is at the slower rate.
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