A project can be completed by A in 25 days and by B in 50 days. Now the two are working together, A left for a few days, and the whole project was completed in 30 days. How many days did A leave on the way?
一项工程，甲单独做要25天完成，乙单独做要50天完成．现在两人合做，甲中途离开了若干天，整个工程30天才完成．甲中途离开了多少天？
<gadget id="calculator">1 / 50</gadget>
<output>1/50 = around 0.02</output>

<gadget id="calculator">(1/50) * 30</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 / 25</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(2/5) / (1/25)</gadget>
<output>10</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<result>20</result>