Question

Question 3 Abdul, 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 Abdul’s trip?

Class 9 - Science - Motion Page 112

Answer 1

Let the distance to the school be x

Total distance = 2x
Total time = x/20 + x/30
=> 5x/60

Average speed = Total distance / Total time taken

=> 2x / (5x/60)
=> 2x × 60/5x
x will cancel out
=> 2 × 60/5
=> 24 km/hr


ANSWER = 24 km/hr

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Answer 2

Hey friend!
___________

Here is your answer
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Case 1:

Average speed = 20 km/hr

Average speed = Distance/time

Time = distance/speed

t(1) = d/20

Case 2:

Average speed = 30 km/hr

Average speed = distance/time

Time = distance/speed

t(2) = d/30

Average speed for whole trip :

Total distance/total time

Total distance = d + d = 2d

$\frac{2d}{ \frac{d}{20} + \frac{d}{30} }$

$\frac{2d}{ \frac{d}{12} }$

$\frac{24d}{d}$

= 24 km/hr
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HOPE THIS HELPS YOU :)