A project can be completed in 10 days by A alone, 12 days by B alone, and 15 days by C alone. Now, after A and B cooperate for 4 days, the rest of the project will be done by C alone. How many days will it take? ?
一项工程，甲单独做需要10天完成，乙单独做需要12天完成，丙单独做需要15天完成，现在甲、乙合作4天后，剩下的工程由丙独做，还需要多少天完成？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) + (1/12)</gadget>
<output>11/60 = around 0.183333</output>

<gadget id="calculator">(11/60) * 4</gadget>
<output>11/15 = around 0.733333</output>

<gadget id="calculator">1 - (11/15)</gadget>
<output>4/15 = around 0.266667</output>

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

<gadget id="calculator">(4/15) / (1/15)</gadget>
<output>4</output>

<result>4</result>