batch of parts can be completed by A alone in 15 days, and by B alone in 20 days. Now A and B have cooperated for 12 days to complete the project. How many days did B rest during this period?
一批零件，甲单独做15天完成，乙单独做20天完成．现由甲、乙合作12天就完工了，这段时间里乙休息了几天？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 12</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/5) / (1/20)</gadget>
<output>4</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<result>8</result>