To process a batch of parts, it takes 6 days for group A to do it alone, 9 days for group B to do it alone, and 18 days for group C to do it alone. If two groups A and B are allowed to work together, after 2 days of cooperation, there are still 56 unfinished parts. If group C works alone, how many days will it take to complete this batch of parts?
加工一批零件，甲组独做需要6天，乙组独做需要9天，丙组独做每天可做18个．如果让甲、乙两组合作，则合做2天后，还剩下56个零件没有完成，如果由丙组独做，完成这批零件需要多少天？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/6) + (1/9)</gadget>
<output>5/18 = around 0.277778</output>

<gadget id="calculator">2 * (5/18)</gadget>
<output>5/9 = around 0.555556</output>

<gadget id="calculator">1 - (5/9)</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">56 / (4/9)</gadget>
<output>126</output>

<gadget id="calculator">126 / 18</gadget>
<output>7</output>

<result>7</result>