It takes 20 hours for the master to process a batch of parts, and 30 hours for the apprentice to do it alone. Now the master and apprentice cooperate for 4 hours, and the rest is done by the master alone. How many hours will it take?
加工一批零件，师傅独做20小时完成，徒弟独做要30小时完成，现在师徒合作4小时，剩下的由师傅独做，还要多少小时完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 4</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">(2/3) / (1/20)</gadget>
<output>40/3 = around 13.333333</output>

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