A car travels the first one third of a certain distance with a speed of 10 km/hr, the next one third with a speed of 20 km/hr and the last one third distance with a speed of 60 km/hr The average speed of the car for the whole journey is

18 km/hr

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24 km/hr

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30 km/hr

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36 km/hr

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Solution

The correct option is A 18 km/hr
Let the total distance travelled be x km Then total time taken = $\frac{x / 3}{10} + \frac{x / 3}{20} + \frac{x / 3}{60} = \frac{x}{30} + \frac{x}{60} + \frac{x}{180}$
$= \frac{6 x + 3 x + x}{180} = \frac{10 x}{180} = \frac{x}{18}$ hrs Then average speed = $= \frac{T o t a l d i s t a n c e t r a v e l l e d}{T o t a l t i m e t a k e n}$
$= \frac{x}{x / 18}$ km/hr = 18 km/hr

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A car moving on a straight road covers one third of the distance with 20 km/hr and the rest with 60 km/hr. The average speed is

20 k m / h r

60 k m / h r

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