A project can be completed by A alone in 20 days, and by B in 15 days. Now A and B cooperate for a few days and then B will do it alone. It takes 6 days successively. How many days does A do?
一项工程，甲独做20天完成，乙15天完成，现在甲乙合作若干天后剩下由乙独做，先后用6天，甲做了多少天？
<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>