Xiaohong travels 15 kilometers per hour from place A to place B, and travels 10 kilometers per hour from place B back to place A. What is the average speed of his round trip in kilometers per hour?
小红从甲地到乙地每小时行15千米，从乙地返回甲地每小时行10千米，他往返一次的平均速度是多少千米/小时．
<gadget id="calculator">30 + 30</gadget>
<output>60</output>

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

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

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

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

<result>12</result>