There is a job that A alone takes 12 days to complete, and B alone takes 15 days to complete. A first does it for 3 days, and then A and B finish it together. Then, how many days did it take from the beginning to the end of the job?
有一项工作甲独做需12天完成，乙独做需15天完成，甲先做了3天，然后甲乙两人共同完成，那么，这项工作从开始到结束共用了多少天．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/12) + (1/15)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/4) / (3/20)</gadget>
<output>5</output>

<gadget id="calculator">3 + 5</gadget>
<output>8</output>

<result>8</result>