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, and this is done alternately, it happens to be completed in an integer number of days. If B does it on the first day, A does it on the second day, In this way, it takes half a day longer to complete the work in turn than the last time. How many days will it take for A to complete this task alone?
一项工程，乙单独做20天可以完成，如果第一天甲做，第二天乙做，这样轮流交替做，恰好也用整数天完成，如果第一天乙做，第二天甲做，这样轮流交替做，比上次轮流做要多做半天才能完成，这项工作甲单独完成需要几天？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<result>10</result>