If a project is completed alone, A will take 20 hours and B will take 30 hours. If two people work together, how many hours can it be completed?
一项工程如果单独完成，甲要20小时，乙要30小时．如果两人合做，几小时可以完成？
<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>