There is a hill in East Smurf Village. On this hill is a trail. Nobody knows how long the trail is. One morning, Papa Smurf drives his car up the trail at an average speed of 40 miles per hour. Arriving at the top, he takes a negligible amount of time to turn around. (This means that the time taken for turn-around is so small compared to the time taken for the trip itself that it can be neglected without any noticeable loss of accuracy.) Papa Smurf then drives his vehicle down the exact same trail. On the way down, he drives at an average speed of 60 miles per hour. For the entire round-trip journey, what is Papa Smurf's average speed? Please show work