To build a road, Zhang San needs 20 days to complete, and Li Si needs 30 days to complete. If two people cooperate, how many days will it be completed?
修一条路，张三需要20天完成，李四需要30天完成．若两人合作，几天完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

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

<result>12</result>