A job is done by A alone for 20 hours, and B alone for 12 hours. Now A alone will do it for 4 hours, and A and B will do the rest together. How many hours does the rest take?
一件工作，甲单独做20个小时完成，乙单独做12小时完成，现在先由甲单独做4小时，剩下的部分由甲、乙合做．剩下的部分需要几小时完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 4</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">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/20) + (1/12)</gadget>
<output>2/15 = around 0.133333</output>

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

<result>6</result>