A car travels the first one third of a certain distance with a speed of 10 km hr the next one third ...........

Quantitative Aptitude (CDS) Time & distance

A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance 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

24 km/hr

30 km/hr

36 km/hr

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Explanation:

Let the distance be 3x

Average speed = (total distance traveled) / (total time taken)
So, let times taken for the first, second and third halves of the journeys be t1, t2, and t3 respectively.
So,
t₁=x/10
t₂=x/20
t₃=x/60
So, average speed = $\frac{3x}{\frac{x}{10}+\frac{x}{20}+\frac{x}{60}}$ = $\frac{3}{\frac{6+3+1}{10}}$