Xiaojun went to climb the mountain. He traveled 2.5 kilometers per hour when he went up the mountain, and 4 kilometers per hour when he went down the mountain and returned by the same route. The round trip took 3.9 hours. How many kilometers did the small army travel back and forth?
小军去爬山，上山每小时行2.5千米，下山按原路返回时每小时行4千米，往返共用3.9小时．小军往返共行多少千米？
<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>