It is an uphill road from A to B. Xiao Ming travels from A to B in the morning at a speed of 12 kilometers per hour, and in the afternoon Xiao Ming returns at a speed of 20 kilometers per hour. What is the average speed of his round trip in kilometers per hour?
从A到B地是一段上坡路，小明上午从A到B，速度是12千米每小时，下午小明返回的速度为20千米每小时，则他往返的平均速度是多少千米每小时．
<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>

<result>15</result>