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 5 kilometers per hour. What is the average speed for the round-trip in kilometers per hour? Choose the correct option
A) 5.5
B) 7.1
C) 7.2
D) 7.0
E) 8.0

Rationale: "pick a number which is lcm of 5 and 6 = 30 . upstream time = 30 / 6 = 5 hrs downstream time = 30 / 5 = 6 hrs total time = 11 hrs total distance = 60 average speed = 60 / 11 = 5.5 km / hr"
Answer: A