A piece of work is done jointly by A and B in 12 days, A alone in one day, and B alone in 3 days (3/20). If B does it alone, how many days will it be completed?
一件工作，甲乙合做，12天完成，甲独做一天，乙独做3天完成(3/20)，如果由乙独做，几天完成？
<gadget id="calculator">3 / 20</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

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

<gadget id="calculator">(3/20) - (1/12)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) / 2</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / (1/30)</gadget>
<output>30</output>

<result>30</result>