For a project, A alone takes 15 days to complete, and B alone takes 20 days to complete. After the two work together for a few days, A has another task to transfer away, and the remaining project B takes another 6 days to complete. The two teams work together How many days?
一项工程，甲独做要15天完成，乙独做要20天完成，两人合做若干天后，甲另有任务调走，余下的工程乙又用了6天时间完成，两队合做了多少天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 6</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/10) / (7/60)</gadget>
<output>6</output>

<result>6</result>