The master and apprentice work together to complete a job, and the apprentice does it alone in 20 days, which is 4 days longer than the master. If the master and apprentice cooperate, how many days will it take to complete it? (Yinchuan Experimental Primary School)
师徒共同完成一件工作，徒弟独做20天完成，比师傅多用4天完成，如果师徒合作需几天完成？(银川市实验小学)
<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">1 / 16</gadget>
<output>1/16 = around 0.0625</output>

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

<gadget id="calculator">(1/16) + (1/20)</gadget>
<output>9/80 = around 0.1125</output>

<gadget id="calculator">1 / (9/80)</gadget>
<output>80/9 = around 8.888889</output>

<result>80/9 = around 8.888889</result>