There is a batch of parts that A alone can complete in 10 hours, B alone can complete it in 12 hours, and C alone can complete it in 15 hours. Now, A and B can work together for 4 hours, and C can do the rest by himself. How many more hours are needed? Finish?
有一批零件，甲独做10小时完成，乙独做12小时完成，丙独做15小时完成，现在先由甲乙两人合做4小时，余下的由丙一个人独做，还要几小时可以完成？
<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>