The correct option is D 24 km/h
Let the total distance from the house to the school be x km. The speed of the bus to and from the school is 20 km/h and 30 km/h, respectively. We can calculate the time taken for going to the school and coming back, as follows:
Time taken for going =x/20 h
Time taken for coming back
=x/30h
Total time taken for the entire journey =(x/20+x/30)h
Total distance for the entire journey =x+x=2x
Formula for average speed = Total distance travelled / Total time taken
Therefore,average speed =(2x)/(x/20+x/30)
2x/(5x/60)=24km/h