How to solve for average velocity on a distance time graph?

[justinh8]

Homework Statement

Hi i need some help with this question: A teacher drives to work at a average velocity of 80km/h. He forgets the test and INSTANTLY turns around and travels back home at 120km/h. What is the average speed for the trip?
Do i have to find instantaneous velocity? I am not really sure and need some help

 

[drnickriviera]

The teacher covers the same distance both ways. You know that the distance he covered in all is twice that, and the time it took him each way can be obtained in terms of that distance. The average speed will just be the ratio of the total time to the total distance.