To process a batch of parts, A alone can complete it in 9 days, and B alone can complete it in 12 days.
加工一批零件，甲单独做9天完成，乙单独做12天完成，两人合做多少天完成．
<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

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

<gadget id="calculator">(1/9) + (1/12)</gadget>
<output>7/36 = around 0.194444</output>

<gadget id="calculator">1 / (7/36)</gadget>
<output>36/7 = around 5.142857</output>

<result>36/7 = around 5.142857</result>