A project alone takes 30 days to complete, and B alone takes 20 days to complete. After the two worked together for 4 days, because A temporarily left for other tasks, B worked alone for 5 days. The rest of the work will be done by two people together, and how many days will it take to finish.
一项工程，甲单独做要30天完成，乙单独完成要20天完成．两人一起合做了4天后，因为甲临时有其他任务离开，由乙单独做了5天．剩下的工作由两人一起做，还需要多少天能做完．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/30) * 4</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">(1/20) * 9</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">1 - (2/15) - (9/20)</gadget>
<output>5/12 = around 0.416667</output>

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

<gadget id="calculator">(5/12) / (1/12)</gadget>
<output>5</output>

<result>5</result>