Xiao Ming goes to climb a mountain, and travels 2.5 kilometers per hour when going up the mountain, and 4 kilometers per hour when going down the mountain, and the round trip takes 3.9 hours. How many kilometers did Xiao Ming travel for a round trip?
小明去爬山，上山时每时行2.5千米，下山时每时行4千米，往返共用3.9时．小明往返一趟共行了多少千米？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(2/5) + (1/4)</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">3.9 / (13/20)</gadget>
<output>6</output>

<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<result>12</result>