There are two jobs, A and B. It takes Zhang 10 days to complete A alone, and 15 days to complete B alone; Li takes 8 days to complete A alone, and 20 days to complete B alone. If each job can be done by two How many days will it take to complete both tasks.
有甲、乙两项工作，张单独完成甲工作要10天，单独完成乙工作要15天；李单独完成甲工作要8天，单独完成乙工作要20天，如果每项工作都可以由两人合作，那么这两项工作都完成最少需要多少天．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 8</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 / 20</gadget>
<output>1/20 = around 0.05</output>

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

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

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

<result>12</result>