A motorcyclist drives from A to B with a uniform speed of 30, km, h^{-1} and returns back with a spe

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7. MOTION

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A motorcyclist drives from $A$ to $B$ with a uniform speed of $30\, km\, h^{-1}$ and returns back with a speed of $20 \,km \,h^{-1}$. Find its average speed(in $km\, h^{-1}$).

A

$20$

B

$30$

C

$34$

D

$24$

Solution

Let $AB =x,$ So ${ t }_{ 1 }=, \frac{x}{30}$ and $t_{2}=\frac{x}{20}$

Total time $=t_{1}+t_{2}=\frac{5 x}{60}\, h$

Average speed for entire journey $=\frac{\text { Total distance }}{\text { Total Time }}=\frac{2 x}{\frac{5 x}{60}}=24 \,km\,h ^{-1}$

Standard 9

Science

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