There are two projects A and B. It takes Li Ming 9 days to complete project A alone, and 12 days to complete project B alone. It takes Wang Gang 3 days to complete Project A alone, and 15 days to complete Project B alone. How many days would it take for two people to work together to complete the two projects?
有甲乙两项工程，李明单独完成甲工程需要9天，单独完成乙工程要12天．王刚单独完成甲工程需要3天，单独完成乙工程要15天．如果两人合作完成这两项工程需要多少天．
<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>