It takes 15 days for A and B to process a batch of parts together. Now A will leave after processing for 12 days, and B will continue to process for 20 days to complete the task. How many days does it take for B to finish processing alone?
加工一批零件，甲乙二人共同加工要15天才能完成．现在甲先加工12天后离开，乙再接着加工20天才完成任务．乙单独加工要多少天才能加工完．
<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>