If a car goes up the mountain at a speed of 12 kilometers per hour and goes down the mountain at a speed of 20 kilometers per hour, what is the average round trip speed in kilometers per hour?
一辆汽车上山的速度是每小时12千米，按原路下山的速度每小时20千米，往返的平均速度是每小时多少千米．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/12) + (1/20)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">2 / (2/15)</gadget>
<output>15</output>

<gadget id="calculator">1 * 15</gadget>
<output>15</output>

<result>15</result>