For a project, A should do it on the first day, B on the second day, A on the third day, and B on the fourth day, so that they can be done alternately, so that the project can be completed in an exact number of days. A will do it, B will do it on the third day, and A will do it on the fourth day. In this way, they will do it alternately, so the completion time will be +. Given that it takes 20 days for B to complete the project alone, how many days will it take for A to complete the project alone?
一项工程，第一天甲做，第二天乙做，第三天甲做，第四天乙做，这样交替轮流做，那么恰好用整数天完工，如果第一天乙做，第二天甲做，第三天乙做，第四天甲做，这样交替轮流做，那么完工时间要+．已知乙单独完成这项工程需要20天，则甲单独完成这项工程需要多少天．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

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

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

<result>10</result>