A project can be completed in 15 days by A alone, and 20 days by B alone. After a few days, A cooperates with B in 4 days, which is just completed. How many days did A work before A and B cooperated?
一项工程，甲单独做15天完成，乙单独做20天完成，甲做若干天后，再与乙合作4天，正好完成，甲乙合作前，甲做了多少天？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">(7/60) * 4</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">1 - (7/15)</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">(8/15) / (1/15)</gadget>
<output>8</output>

<result>8</result>