project can be completed in 9 days if A and B work together; if B and C work together, it can be completed in 12 days. Now this project took 18 days to complete, because A only did it in 3 days, B did it in 9 days, and C persisted from the beginning to the end. Ask if this project is completed by A, B, and C, how many days will it take?
一件工程，如果甲、乙两人合做，9天可完成；如果乙、丙两人合做，12天可完成．现这项工程共用18天才完成，因为甲只做了3天，乙做了9天，丙从开始坚持到结束．问如果这项工程由甲、乙、丙三人自始至终合作完成，需要多少天？
<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">3 * (1/9)</gadget>
<output>1/3 = around 0.333333</output>

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

<gadget id="calculator">6 * (1/12)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/3) - (1/2)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) / 12</gadget>
<output>1/72 = around 0.013889</output>

<gadget id="calculator">(1/12) - (1/72)</gadget>
<output>5/72 = around 0.069444</output>

<gadget id="calculator">(1/9) - (5/72)</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">(1/72) + (5/72) + (1/24)</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">1 / (1/8)</gadget>
<output>8</output>

<result>8</result>