A project can be completed by B alone in 20 days. If A does it on the first day, and B does it on the second day, it will take an integer number of days to do it alternately; if B does it on the first day, and A does it on the second day, it will take half a day longer to complete it than the last time. ． How many days can this project be completed by A alone?
一项工程，乙单独做20天完成．如果第一天甲做，第二天乙做，这样交替做也恰好用整数天完成；如果第一天乙做，第二天甲做，这样交替做结果比上次交替做要多半天才能完成．这项工程由甲单独做需要几天可以完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 2</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / (1/10)</gadget>
<output>10</output>

<result>10</result>