Xiaoling rides a bicycle across a bridge. The speed on the bridge is 12 km/h, and the speed off the bridge is 24 km/h. The distance traveled by the upper and lower bridges is the same. What is the average speed of crossing the bridge in km/h?
小玲骑车过一座桥，上桥经过速度为12千米/小时，下桥速度为24千米/小时，上下桥经过的路程相同，过桥的平均速度是多少千米/小时．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

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

<gadget id="calculator">1 / 24</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">(1/12) + (1/24)</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">2 / (1/8)</gadget>
<output>16</output>

<result>16</result>