It takes 20 hours for the master to produce a batch of parts, and 25 hours for the apprentice. Now the master and the apprentice are cooperating, and the master has something to go out halfway, so it took 15 hours to complete. How many hours did the master do it?
生产一批零件，师傅要做20小时，徒弟要做25小时，现在师傅和徒弟合作，中途师傅有事外出，结果用了15小时完成．师傅做了多少小时？
<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>