certain job can be completed by A alone for 45 days, and then by B alone for 18 days. If A and B cooperate, they can complete it in 30 days. Now A will do it alone for 20 days first, and then B will finish it alone. How many days will it take?
某项工作先由甲单独做45天，再由乙单独做18天可以完成，如果甲乙两人合作可30天完成．现由甲先单独做20天，然后再由乙来单独完成，还需要多少天．
<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<gadget id="calculator">45 - 30</gadget>
<output>15</output>

<gadget id="calculator">30 - 18</gadget>
<output>12</output>

<gadget id="calculator">15 / 12</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">10 / (5/4)</gadget>
<output>8</output>

<gadget id="calculator">30 + 8</gadget>
<output>38</output>

<result>38</result>