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 3 kilometers per hour. What is the average speed for the round-trip in kilometers per hour? Choose one.
A) 4.0	B) 3.8	C) 7.2	D) 7.5	E) 8.0

Rationale: "pick a number which is lcm of 3 and 6 = 12 . upstream time = 12 / 6 = 2 hrs downstream time = 12 / 3 = 4 hrs total time = 6 hrs total distance = 24 average speed = 24 / 6 = 4.0 km / hr"
Answer: A