Abdul while driving to school, computes the average speed for his trip to be 20 km `h^(-1)`. On this trip along the same route there os less traffic and average speed is 40 km `h^-(-1) . What is the average speed for Abdul's trip ?

Explanation

Let the distance between starting point and school be x km.
Average speed from the starting point to school
`="20 km h"^(-1)`
Time for onward journey `=(x)/(20)h`
Average speed from the school to the starting point
`=" 40 km h"^(-1)`
Time for return journey `=(x)/(40)h`
Average speed for total trip
`=("Total distance")/("Total time")`
`=(2x)/((x)/(20)+(x)/(40))`
`=(2x)/((21x)/(40))`
`=2x xx(40)/(21x)=(80)/(21)=3.8" km/h"`