A project alone will take 20 days to complete. Now A alone will complete it in 4 days, and A and B will cooperate to complete the task in 6 days. If B alone does this project, how many days will it take to complete it?
一项工程，甲单独做要20天完成，现由甲单独做了4天以后由甲乙两人合作，再做6天就完成任务，如果这项工程由乙单独做，要做多少天完成？
<gadget id="calculator">4 / 20</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) / 6</gadget>
<output>2/15 = around 0.133333</output>

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

<gadget id="calculator">(2/15) - (1/20)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / (1/12)</gadget>
<output>12</output>

<result>12</result>