Alex rode his bike to school at a speed of 12 mph. He then walked home at a speed of 5 mph. What was

Alex rode his bike to school at a speed of 12 mph. He then walked home at a speed of 5 mph. What was Alex's average speed for his trip to school and back?

Say the distance was 1 mile from school to home
D = rt

To school
1 = 12t
t = 1/12

From school:
1 = 5t
t = 1/5

1/2(1/12 + 1/5)
1/24 + 1/10 = 17/120

120 = Average speed * 17
Average speed = 120/17 = 7.06 mph