A black and white team climbs a mountain at a speed of 6 km/hour up the mountain and 12 km/hour down the mountain by the same route. What is the average speed of the black and white team going back and forth in kilometers/hour?
黑白团队爬一座山，上山的速度是6千米/每小时，按原路下山的速度是12千米/每小时，黑白团队往返的平均速度是多少千米/每小时．
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/6) + (1/12)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 / (1/4)</gadget>
<output>8</output>

<result>8</result>