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 one:
A) 5.1 B) 7.1 C) 7.2 D) 7.5 E) 8.0
Pick a number which is LCM of 7 and 4 = 18.   Upstream time = 28/4= 
<gadget id="calculator">28/4</gadget>
<output>7</output>
7 hrs   Downstream time= 28/7= 
<gadget id="calculator">28/7</gadget>
<output>4</output>
4 hrs   Total time= 11hrs   Total distance = 56   5.1verage Speed = 56/11 = 5.1 km/hr
<result>A</result>
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?