David is a trail runner who decides to take a day off work to run up and down a local mountain. He runs uphill at an average speed of 3 miles per hour and returns along the same route at an average speed of 8 miles per hour. Of the following, which is the closest to his average speed, in miles per hour, for the trip up and down the mountain?	Pick one:
A) 4.3  B) 5.8  C) 6.0  D) 6.3  E) 6.5
to calculate average of 2 speeds a and b when distance is constant   Formula - 2ab/(a+b)   Here David's uphill speed = 3 mph   David's downhill speed = 8 mph   2*3*8/(3+8) = 48/11 = 4.3   Correct answer - 4.3
<result>A</result>
David is a trail runner who decides to take a day off work to run up and down a local mountain. He runs uphill at an average speed of 3 miles per hour and returns along the same route at an average speed of 8 miles per hour. Of the following, which is the closest to his average speed, in miles per hour, for the trip up and down the mountain?