During the holiday, Yaoyao's family went for an outing by bicycle. The time they went was 15 kilometers per hour, and the time when they returned was 10 kilometers per hour. What is the average speed of their round trip in kilometers per hour?
假日，遥遥一家骑车去郊游，去时每小时15千米，返回时每小时10千米，他们往返的平均速度是每小时多少千米．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/15) + (1/10)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">2 / (1/6)</gadget>
<output>12</output>

<result>12</result>