There is an uphill road with a total round trip distance of 120 kilometers. Kobayashi rides a bicycle uphill at 10 kilometers per hour and downhill at 15 kilometers per hour. Find the average speed of the bicycle.
一段上坡路，往返路程共120千米，小林骑车上坡每小时行10千米，下坡每小时行15千米，求自行车的平均速度．
<gadget id="calculator">120 / 2 / 10</gadget>
<output>6</output>

<gadget id="calculator">120 / 2 / 15</gadget>
<output>4</output>

<gadget id="calculator">6 + 4</gadget>
<output>10</output>

<gadget id="calculator">120 / 10</gadget>
<output>12</output>

<result>12</result>