<p>Paul drives from his apartment to his parents' house and back. On the trip to his parents' house, he travels at an average speed of 60 miles per hour. On the return trip, Paul drives at an average speed of 80 miles per hour. Which of the following is the closest approximation of Paul's average speed, in miles per hour, for the round trip?</p>

<p>A. 60

B. 68.6

C. 70

D. 71.4

E. 80</p>

<p>No idea what the answer is, and the book doesn't tell me either. It also says "Don't pick obvious answers (obvious answer being C).</p>