The distance between A and B is 90 kilometers. A person rides a bicycle downhill from A and travels an average of 18 kilometers per hour. On the way back, he goes uphill and travels an average of 9 kilometers per hour. Find the round-trip distance between the two places What is the average speed in kilometers?
甲乙两地相距90千米，某人骑自行车从甲地一直下坡，平均每小时行18千米，返回时爬坡，平均每小时行9千米，求此人在两地间往返行程的平均速度是多少千米？
<gadget id="calculator">90 * 2</gadget>
<output>180</output>

<gadget id="calculator">90 / 18</gadget>
<output>5</output>

<gadget id="calculator">90 / 9</gadget>
<output>10</output>

<gadget id="calculator">5 + 10</gadget>
<output>15</output>

<gadget id="calculator">180 / 15</gadget>
<output>12</output>

<result>12</result>