please solve

6. (8p.) You drive from San Antonio to Houston, one-third the time at 40 mi/h, the second third at 60 mi/h, and the last third at 20mi/h. On the way back you travel one- third the distance at 40 mi/h, the second third at 60mi/h, and the last third at 20 mi/h. What is your average speed a) from San Antonio to Houston, b) from Houston back to San Antonio, and c) for the entire trip