The distance from Maria's home to school is d.
The distance covered by Maria when she drove to school and then back home is then D = d + d = 2d
Total time taken by Maria to cover this distance is t = 20 minutes + 24 minutes = 44 minutes
Average speed is given as
v = D/t
v = (2d)/(44 minutes)
v = d/22 units/minutes
Therefore, Maria's average speed is d/22 units/min.