What is the average speed for the entire trip?

[napolij]

Homework Statement

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 ?

Homework Equations

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

 

[PhanthomJay]

His average speed for the trip can't be greater than 40 km/hr, can it? His total time is 24 minutes to some point, 24 minutes back, 9 minute delay, and then when he starts out again, he travels the 40 km distance at 40 km/hr, so his time for the last part of the trip is not 24 minutes, it is ____ minutes. Now add them up for the total time. Now for the total distance, you are way off, as you seem to be summing speeds instead of summing distances. ? How far does he travel in 24 minutes east, and then in 24 minutes west? And then how far does he travel in the final leg of the journey (which is given).

 

[bentleyq]

4 Legs,
1st, 40km/h - 24min(.4hr) - 9.6km
2nd, 40km/h - 24min(.4hr) - 9.6km
3rd, 0km/h - 9min(.15hr) - 0km
4rth, 40km/h - 1hr - 40km