For a project, engineering team A worked for 4 days first, and completed (1/5) of the project. Engineering team B also participated in the work together, and it took a total of 6 days to complete the entire project. If this project is handed over to two engineering teams, A and B, to start and cooperate at the same time, how many days will it take?
一项工程，甲工程队先做4天，完成了工程的(1/5)，乙工程队也参加一起做，又共做了6天才完成全部工程．如果这项工程交给甲乙两个工程队同时开工合作完成，需要几天？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) / 6</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1 / (2/15)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>