There are two projects A and B. It takes Zhang San 12 days to complete project A alone, and 15 days to complete project B alone; Li Si needs 8 days to complete project A alone, and 20 days to complete project B alone. How many days will it take at least for Zhang San and Li Si to complete the project together?
有甲乙两个工程，张三单独做完甲工程需要12天，单独做完乙工程需要15天；李四单独做完甲工程需要8天，单独做完乙工程20天．张三李四二人共同完成这个工程最少需要多少天？
<gadget id="calculator">8 / 15</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">1 - (8/15)</gadget>
<output>7/15 = around 0.466667</output>

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

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/15) / (7/60)</gadget>
<output>4</output>

<gadget id="calculator">8 + 4</gadget>
<output>12</output>

<result>12</result>