It takes 15 days for the master to process a batch of parts, and 20 days for the apprentices alone. After the apprentice does it alone for a few days, the master will do the rest for 5 days alone to complete the task. How many days did the apprentice do alone first?
加工一批零件，师傅单独要做15天完成，徒弟单独要做20天完成．徒弟先单独做若干天后，剩下的再由师傅单独做5天才能完成任务．徒弟先单独做了多少天？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 5</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

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

<gadget id="calculator">(2/3) / (1/20)</gadget>
<output>40/3 = around 13.333333</output>

<result>40/3 = around 13.333333</result>