physics
posted by ashley .
A student wanted to drive from Austin to san Antonio ,80 mi south of Austin on highway I35. Unfortunately he entered in the wrong direction and drove to waco100 miles north of Austin –before he noticed his error. In waco ,he turned around,drove back to Austin and continued to San Antonio . The whole trip took 5.46 h.
What was the student’s average speed during this trip? Answer in units of mph
What was the student’s average velocity during this trip? Take your positive direction to be southbound on I 35.

distance= 80+2*100=280
speed=distance/time
veloctiy= 80/time Southbound 
Would the time southbound be if I were to half 5.46? Or, I looked it up that it would take about 1 h 30 min from austin to san antonio.
I ended up taking 80mi/1.5h=53mph.
If that's not a good way to go about it, how do I figure out the time southbound? 
I also tried 180mi/51.3(t)=3.50877193h. Then i took 80mi/that answer to get 22.8mph. Is that the right approach?