A project can be completed in 20 days by A alone, 30 days by B alone, and 12 days by C alone. Now, A and B will work together for 6 days and then C will join in. How many days will it take for the three to complete the project together? project?
一工程，甲独做需20天完成，乙独做需30天完成，丙独做需12天完成，现在先由甲乙合做6天后丙加入，三人合做，还要多少天才能完成这项工程？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

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

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/20) + (1/30) + (1/12)</gadget>
<output>1/6 = around 0.166667</output>

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

<result>3</result>