A river boat leaves Silver Town and travels upstream to Gold Town at an average speed of 6 kilometers per hour. It returns by the same route at an average speed of 8 kilometers per hour. What is the average speed for the round-trip in kilometers per hour?  Choose the correct answer:
A) 6.8
B) 7.1
C) 7.2
D) 7.5
E) 8.0

Rationale: "pick a number which is lcm of 8 and 6 = 24 . upstream time = 24 / 6 = 4 hrs downstream time = 24 / 8 = 3 hrs total time = 7 hrs total distance = 48 average speed = 48 / 7 = 6.8 km / hr"
Answer: A