For a project, it takes 12 days for team A to do it alone, and 15 days for team B to do it alone. If team B works 6 days first and then the two teams work together, how many days will it take to complete the task?
一项工程，甲队单独做要12天，乙队单独做要15天，如果乙队先做6天后两队一起做，还要多少天才能完成这项任务？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 6</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) + (1/15)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/5) / (3/20)</gadget>
<output>4</output>

<result>4</result>