Abdul, while driving to school, computes the average speed for his trip to be 20km/h.  On his return  trip along the same route, there is less traffic and the average speed is 30 km/h . What is the average speed for Abdul’s trip?

 -Shresthi Goyal, Subject Matter Expert at Edumarz 

Answer: 

Case I : Average Speed = Total Distance/Total Time taken 

20 = D/t

t = D/20

Case II : Average Speed  = Total Distance /Total Time Taken

30 = D/T ( Since Distance is same)

T = D/30

Average Speed for whole trip  = Total Distance/Total Time  Taken

Average Speed = 2D /(T + t)

= 2D/(D/20 + D/30)

= 2D×60/5D

=120/5 = 24km/h

Average Speed = 24 km/h