Doudou goes to school by bicycle at 18km/h from home to school, and 12/km/h when returning home from school, so what is the average speed of Doudou to and from school in km/h?
豆豆上学骑车从家里到学校18km/h，从学校回家时12/km/h，那么豆豆往返学校的平均速度是多少km/h．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/18) + (1/12)</gadget>
<output>5/36 = around 0.138889</output>

<gadget id="calculator">2 / (5/36)</gadget>
<output>72/5 = around 14.4</output>

<result>72/5 = around 14.4</result>