It takes 15 days for the master to complete a batch of parts, and 20 days for the apprentice to complete. Now the apprentice makes 6 days first, and then the master and apprentice work together. How many days do they have to work at the same time to complete the task?
一批零件，师傅做要15天完成，徒弟做要20天完成，现在徒弟先做6天，师傅再与徒弟一同工作，他们要同时工作几天才能完成任务？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 6</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/10) / (7/60)</gadget>
<output>6</output>

<result>6</result>