job is completed by A in 12 days alone, B's efficiency is A's (4/5), C's 4 days to complete the whole work (2/5), A, B, C cooperate to complete this work in how many days? (don't use equations to solve)
一项工作，甲单独做12天完成，乙的效率是甲的(4/5)，丙4天完成全部工作的(2/5)，甲、乙、丙合作这项工作几天完成？(不要用方程解答)
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(1/12) * (4/5)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) / 4</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/12) + (1/15) + (1/10)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / (1/4)</gadget>
<output>4</output>

<result>4</result>