A project can be completed in 4 days alone, and B can be completed in 12 days alone. How many days will A and B cooperate to complete the project?
一项工程甲独做4天完成，乙独做12天完成．甲乙合作多少天完成这项工程．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">(1/4) + (1/12)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / (1/3)</gadget>
<output>3</output>

<result>3</result>