A river boat leaves Silver Town and travels upstream to Gold Town at an average speed of 4 kilometers per hour. It returns by the same route at an average speed of 7 kilometers per hour. What is the average speed for the round-trip in kilometers per hour?  Choose the most appropriate option.
A) 5.1 B) 7.1 C) 7.2 D) 7.5 E) 8.0

Rationale: "pick a number which is lcm of 7 and 4 = 18 . upstream time = 28 / 4 = 7 hrs downstream time = 28 / 7 = 4 hrs total time = 11 hrs total distance = 56 average speed = 56 / 11 = 5.1 km / hr"
Answer: A