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 correct choice from the following choices.
A) 4.2  B) 7.1  C) 7.2  D) 7.5  E) 8.0
Pick a number which is LCM of 7 and 3 = 21.   Upstream time = 21/3= 
<gadget id="calculator">21/3</gadget>
<output>7</output>
7 hrs   Downstream time= 21/7= 
<gadget id="calculator">21/7</gadget>
<output>3</output>
3 hrs   Total time= 10 hrs   Total distance = 42   4.2verage Speed = 42/10= 
<gadget id="calculator">42/10</gadget>
<output>21/5 = around 4.2</output>
4.2 km/hr
<result>A</result>
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?