The distance from A to B is 60 km. Xiaohong rides a bicycle from A to B at 15 kilometers per hour, and from B to A at 10 kilometers per hour. Find Xiaohong's average round-trip speed.
甲地到乙地的全程是60千米．小红骑自行车从甲地到乙地每小时行15千米，从乙地到甲地每小时行10千米．求小红往返的平均速度．
<gadget id="calculator">60 * 2</gadget>
<output>120</output>

<gadget id="calculator">60 / 15</gadget>
<output>4</output>

<gadget id="calculator">60 / 10</gadget>
<output>6</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<gadget id="calculator">120 / 10</gadget>
<output>12</output>

<result>12</result>