person goes from East Station to West Station, travels 15 kilometers per hour when going, and travels 10 kilometers per hour when returning, find the average speed of the round trip.
某人从东站到西站，去时每小时行15千米，返回时每小时行10千米，求往返的平均速度．
<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>