The production of a batch of parts was completed by the master alone 3 days earlier than the planned time, and by the apprentice 5 days ahead of the planned time. Now the master and apprentice work together for 3 days, and the apprentice continues to complete the work alone within the planned time. How many days can the master-student cooperation be completed?
生产一批零件，师傅独做比计划时间提前3天完成，徒弟独做超过计划5天完成，现在师徒两人合作3天，徒弟接着独做在计划时间内完成．如果师徒合作几天能够完成？
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">15 + 15</gadget>
<output>30</output>

<gadget id="calculator">5 - 3 - 1 - 3</gadget>
<output>-2</output>

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

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

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

<gadget id="calculator">(1/12) + (1/20)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1 / (2/15)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>