A river boat leaves Silver Town and travels upstream to Gold Town at an average speed of 3 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) 4.2
B) 7.1
C) 7.2
D) 7.5
E) 8.0

Rationale: pick a number which is lcm of 7 and 3 = 21 . upstream time = 21 / 3 = 7 hrs downstream time = 21 / 7 = 3 hrs total time = 10 hrs total distance = 42 average speed = 42 / 10 = 4.2 km / hr
Answer: A