The two masters and apprentices work together to process a batch of parts and complete (1/4) in 3 days. It is known that the master alone needs 20 days to complete. How many days does it take for an apprentice to do it alone?
师徒两人共同加工一批零件，3天完成了(1/4)，已知师傅独做需要20天完成．徒弟独做需要多少天完成？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) / 3</gadget>
<output>1/12 = around 0.083333</output>

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

<gadget id="calculator">(1/12) - (1/20)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / (1/30)</gadget>
<output>30</output>

<result>30</result>