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(A) Let the total distance be $S$ and the average speed be $v_{avg}$.Time taken for the first one-third distance $(S/3)$ at $10\, km/hr$ is $t_1 = \fr

Vedclass · Accepted Answer

$18$

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(A) Let the total distance be $S$ and the average speed be $v_{avg}$.Time taken for the first one-third distance $(S/3)$ at $10\, km/hr$ is $t_1 = \frac{S/3}{10} = \frac{S}{30}$.Time taken for the next one-third distance $(S/3)$ at $20\, km/hr$ is $t_2 = \frac{S/3}{20} = \frac{S}{60}$.Time taken for the last one-third distance $(S/3)$ at $60\, km/hr$ is $t_3 = \frac{S/3}{60} = \frac{S}{180}$.Total time $T = t_1 + t_2 + t_3 = \frac{S}{30} + \frac{S}{60} + \frac{S}{180} = S \left( \frac{6+3+1}{180} ight) = \frac{10S}{180} = \frac{S}{18}$.Average speed $v_{avg} = \frac{	ext{Total Distance}}{	ext{Total Time}} = \frac{S}{S/18} = 18\, km/hr$.

$A$ 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 (in $km/hr$):

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