A mountain road is 30 kilometers long. A motorcycle goes up the mountain at 10 kilometers per hour and goes down the mountain at 15 kilometers per hour. Find the average speed of the motorcycle going up and down the mountain?
一条山路长30千米，一辆摩托车上山每小时10千米，从原路下山每小时15千米，求这辆摩托车上山和下山的平均速度？
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">30 / 10</gadget>
<output>3</output>

<gadget id="calculator">30 / 15</gadget>
<output>2</output>

<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<gadget id="calculator">60 / 5</gadget>
<output>12</output>

<result>12</result>