A project can be completed by A alone in 15 days and by B in 20 days. A works alone for several days, and then cooperates with B for 6 days, just to complete the project. Question: How many days did A work alone before A and B worked together?
一项工程，甲独做15天可以完成，乙独做20天可以完成．甲先单独做若干天后，再与乙合作6天，正好完成这项工程．问：甲、乙合做之前，甲单独做了多少天？
<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) * 6</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(3/10) / (1/15)</gadget>
<output>9/2 = around 4.5</output>

<result>9/2 = around 4.5</result>