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  4. The average speed of a car running at the rate of 15 km/hour during the first 30 km and at 20 km/hour during the second 30 km and at 25 km/hour during the third 30 km, will be

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Q. The average speed of a car running at the rate of 15 km/hour during the first 30 km and at 20 km/hour during the second 30 km and at 25 km/hour during the third 30 km, will be

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Solution:

Here, the required average speed is equal to the harmonic mean of the three speeds
$ = \frac{3}{\frac{1}{15} + \frac{1}{20}+ \frac{1}{25}} $ km/hour
$= \frac{3\times300}{20+15+12} = \frac{900}{47} $ km/hour = 19.15 km/hour.