It takes 15 days for the master and apprentice to process a batch of parts together. If the master does 10 days first, the remaining work will take 17 days for the apprentice to complete alone. How many days will it take for the apprentice to process this batch of parts alone?
师徒两人合加工一批零件需15天才能完成，若师傅先做10天，则剩下的工作，徒弟单独做还需17天才能完成，徒弟单独加工这批零件需要多少天才能完成？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 10</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 - (2/3)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">17 - 10</gadget>
<output>7</output>

<gadget id="calculator">(1/3) / 7</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">1 / (1/21)</gadget>
<output>21</output>

<result>21</result>