Teams A, B, and C have to complete two projects A and B. The workload of Project B is more than that of Project A (1/4). The time required for teams A, B, and C to complete project A independently is 20 days, 24 days, and 30 days, respectively. In order to complete these two projects at the same time, first send team A to do project A, and teams B and C to work on project B together; after a few days, team C and team A are transferred to complete project A together. How many days have you cooperated.
甲、乙、丙三队要完成A、B两项工程．B工程的工作量比A工程的工作量多了(1/4)．甲、乙、丙三队单独完成A工程所需时间分别是20天、24天、30天．为了同时完成这两项工程，先派甲队做A工程，乙、丙两队共同做B工程；经过几天后，又调丙队与甲队共同完成A工程，那么，丙队与乙队合作了多少天．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 24</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">1 + 1 + (1/4)</gadget>
<output>9/4 = around 2.25</output>

<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) + (1/24) + (1/30)</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(9/4) / (1/8)</gadget>
<output>18</output>

<gadget id="calculator">(1/24) * 18</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(1/4) - (3/4)</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">(1/2) / (1/30)</gadget>
<output>15</output>

<result>15</result>