A project can be completed in 20 days alone, and B can be completed in 18 days alone. If A works for a certain number of days first, B will continue to do it immediately, and the task will be completed after 19 days. How many days did A do it first?
一项工程，甲独做20天完成，乙独做18天完成，如果甲先做若干天，乙马上接着做，过19天刚好完成任务．问甲先做了几天？
<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">(1/18) * 19</gadget>
<output>19/18 = around 1.055556</output>

<gadget id="calculator">(19/18) - 1</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/18) - (1/20)</gadget>
<output>1/180 = around 0.005556</output>

<gadget id="calculator">(1/18) / (1/180)</gadget>
<output>10</output>

<result>10</result>