Question

Abdul while driving to school compute the average speed for his trip to be 20km/h on his return trip along the same route with average speed 40km/h what is the average speed foe Abdul trip?

Answer 1

The distance Abdul commutes while driving from Home to School = S

Let us assume time taken by Abdul to commutes this distance = t1
Distance Abdul commutes while driving from School to Home = S
Let us assume time taken by Abdul to commutes this distance = t2
Average speed from home to school v1av= 20 km h-1 Average speed from school to home v2av= 30 km h-1

Also we know Time taken from Home to School t1 =S / v1av Similarly Time taken from School to Home t2 =S/v2av
Total distance from home to school and backward = 2 S

Total time taken from home to school and backward (T) = S/20+ S/30
Therefore, Average speed (Vav) for covering total distance (2S) = Total 
Distance/Total Time = 2S / (S/20 +S/30)
= 2S / [(30S+20S)/600]
= 1200S / 50S
= 24 kmh-1