project can be completed by A alone in 20 days and B by himself in 30 days. A first worked for a few days, and the remaining A and B worked together for 9 days to complete the task. How many days did A do it first?
一项工程，甲单独做20天完成，乙单独做30天完成．甲先做了一些天后，余下的甲、乙合做了9天，完成了任务．甲先做了几天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 9</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) / (1/20)</gadget>
<output>5</output>

<result>5</result>