A bicycle travels 15 kilometers per hour uphill and 30 kilometers per hour back down the same road, what is the average speed of the bicycle going up and down the mountain in kilometers?
一辆自行车，上山每小时行15千米，下山按原路返回每小时行30千米，那么自行车上、下山的平均速度是多少千米．
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/15) + (1/30)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">2 / (1/10)</gadget>
<output>20</output>

<result>20</result>