Uncle Li went on a trip by bicycle. When going there, it traveled 21 kilometers per hour with the wind and arrived in 3 hours; when it came back, it took 4 hours against the wind. What is the average speed of Uncle Li's round trip in kilometers per hour?
李叔叔骑车去旅行．去时顺风每小时行21千米，3小时到达；回来时逆风用了4小时．李叔叔往返的平均速度是每小时多少千米？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">21 * 3 * (2/7)</gadget>
<output>18</output>

<result>18</result>