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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 40 km h-1. What is the average speed for Abdul’s trip?

Answer

Case I: While driving to school

Average speed of Abdul’s trip = 20 km/h

Total distance =  d

Let total time taken = t1

Speed = Distance / time

20 = d / t1

t1 = d / 20…(i)

Case II: While returning from school

Total distance = d

Speed = 30 km/h

Now,total time taken = t2

Speed = distance / time 

30 = d / t2

t2 = d / 30  ..... (ii)

Total distance covered in the trip = d + d = 2d

Total time taken, t = Time taken to go to school + Time taken to return to school

= t1 + t2

Total time taken = d / 20 + d / 30

= 3d + 2d / 60

= 5d / 60

= d / 12

From equations (i) and (ii),

Average Speed = 120/5 

                          = 24  km/h

Hence, the average speed for Abdul’s trip is 24 km/h.

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