A project requires 6 days to be completed by teams A, B, and C, and 9 days to be completed by teams A, B, and 9 days. Now, after 4 days, teams A, B, and B will work together to complete the remaining part. How many days will it take?
一项工程，甲、乙、丙三队合做需要6天完成，甲、乙两队合做需要9天完成，现在由丙队单独做4天后，剩下的部分由甲、乙两队合做还需要多少天？
<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>1/18 = around 0.055556</output>

<gadget id="calculator">(1/18) * 4</gadget>
<output>2/9 = around 0.222222</output>

<gadget id="calculator">1 - (2/9)</gadget>
<output>7/9 = around 0.777778</output>

<gadget id="calculator">(7/9) / (1/9)</gadget>
<output>7</output>

<result>7</result>