A project can be completed in 12 days if A and B work together; if A and B work together in 8 days, B can complete it in 7 days if they work alone. Now B can do it alone for 18 days, and then A can do it alone. How many days will it take to complete it?
一项工程，甲、乙合做12天能完成；如果甲、乙合做8天后，由乙单独做，再做7天也能完成．现由乙单独做18天后，再由甲单独做，还要做多少天才能完成．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 8</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 - (2/3)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/3) / 7</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">(1/21) * 18</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">1 - (6/7)</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/12) - (1/21)</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">(1/7) / (1/28)</gadget>
<output>4</output>

<result>4</result>