Ali while driving to school computes average speed for his trip to be 20, km h^{-1} . On his return

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7. MOTION

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Ali while driving to school computes the average speed for his trip to be $20\, km h^{-1}$. On his return trip along the same route there is less traffic and the average speed is $30\, km h^{-1} .$ What is the average speed for Ali's trip ?

Option A

Option B

Option C

Option D

Solution

Let distance of school from Ali's home be $x$

While going

$\text { Distance }=x, \text { Spet } d =20 km h ^{-1}$

Therefore, time taken

$t_{1}=x / 20 h$

While returning :

$\text { Distance }=x, \text { Speed }=30 km h ^{-1}$

Therefore, time taken

$t_{2}=x / 30 h$

Now, average speed $=$ total distance $/$ total time

$v_{a v}=(x+x) /(x / 20+x / 30)$

$=2 x /(5 x / 60)=24 km h ^{-1}$

Standard 9

Science

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