The distance from Abdullah's house to his school is 2.4km. Abdulla takes 0.6h to go to school on his cycle but takes only 0.4h to return home. His average speed is

The average speed can be calculated as the quotient between the distance travelled and the time needed to travel that distance.

To go to the school, he travels 2.4 km in 0.6 hours, then here the average speed is:

s = (2.4km)/(0.6 hours) = 4 km/h

To return to his home, he travels 2.4km again, this time in only 0.4 hours, then here the average speed is:

s' = (2.4 km)/(0.4 hours) = 6 km/h.

Now, if we want the total average speed (of going and returning) we have that the total distance traveled is two times the distance between his home and school, and the total time is 0.6 hours plus 0.4 hours, then the average speed is:

S = (2*2.4 km)/(0.6 hours + 0.4 hours)

S = (4.8km)/(1 h) = 4.8 km/h