A commuting student leaves home and drives to school at an average speed of 40.0 km/h. After 24.0 min he realizes that he has forgotten his homework and returns home to get it at the same average speed. It takes 9.0 min to find the report, after which the trip to school 40.0 km away to the east is resumed at the same speed as before.
What is the average speed for the entire trip ?
avg speed = total distance/time interval
The Attempt at a Solution
total time = 24+24+24+9 = 81 min = 1.35 hr
total distance = 40+40+40 = 120 km/h
120/1.35 = 88.89 km/h