It takes 15 days for A and B to process a batch of parts. If A does it in 12 days and B does it in 20 days, the task will be completed. How many days does it take B to complete the task alone?
加工一批零件，甲、乙合做需15天完成．如果甲做12天，乙做20天就完成任务．乙单独做需要多少天完成任务？
<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">20 - 12</gadget>
<output>8</output>

<gadget id="calculator">(1/5) / 8</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">1 / (1/40)</gadget>
<output>40</output>

<result>40</result>