A student wanted to drive from Austin to San Antonio, 80 miles south of Austin on highway I35. Unfortunately, he entered the highway in the wrong direction and drove all the way to Waco -- 100 miles north of Austin-- before he noticed his error. In Waco, he turned around, drove back to Austin and continued to San Antonio. What was the student's average velocity during his trip? Take your positive direction to be southbound on I35.