To make a batch of parts, A and B work together to complete it in 12 days. Now A and B work together for 4 days, and the rest can be completed by B alone in 20 days. If A alone completes this batch of parts, how many days will it take?
做一批零件，甲、乙两人合做12天完成，现在甲乙合做4天后，余下的由乙独做20天可以完成，如果甲单独完成这批零件要用多少天？
<gadget id="calculator">1 / 12</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) / 20</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/12) - (1/30)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / (1/20)</gadget>
<output>20</output>

<result>20</result>