One job can be done by A alone in 20 days and B by himself in 25 days. First, A and B will work together, then A will be transferred due to work needs, and B will do it alone. It will take 15 days to complete all the work. How many days does A have to do?
一件工作，甲单独做20天完成，乙单独做25天完成．先由甲乙两人合作，中途甲因工作需要调走，由乙单独干，共计15天才完成全部工作，求甲做了多少天？
<gadget id="calculator">1 / 25</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(1/25) * 15</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">(2/5) / (1/20)</gadget>
<output>8</output>

<result>8</result>