A project is completed in 12 days by the cooperation of A and B, 15 days by the cooperation of B and C, and 20 days by the cooperation of A and C. How many days will it take if A, B and C cooperate to complete it?
一项工程，甲、乙两人合作12天完成，乙、丙两人合作需15天，甲、丙两人合作需20天，如果由甲乙丙三人合作需多少天完成？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

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

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

<gadget id="calculator">(1/15) + (1/12) + (1/20)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) / 2</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / (1/10)</gadget>
<output>10</output>

<result>10</result>