A batch of parts is made individually, A completes it in 6 days, B completes it in 12 days, how many days will the two work together to complete it?
一批零件，单独做，甲6天完成，乙12天完成，两人合做多少天完成．
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

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

<gadget id="calculator">(1/6) + (1/12)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / (1/4)</gadget>
<output>4</output>

<result>4</result>