A job alone takes 20 days and B alone takes 30 days. Now A and B are cooperating, and A is on a business trip for a few days in the middle, and it will be completed after 15 days. How many days does A have to go on business?
一件工作，甲独做要20天，乙独做要30天．现在甲乙合作，中途甲出差了几天，这样经过15天才完成，甲出差了多少天．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/30) * 15</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</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<result>5</result>