To process a batch of parts, A alone takes 12 hours, B alone takes 10 hours, and C alone takes 15 hours. Now B and C cooperate for 4 hours, and A will do the rest alone. How many hours will A need to complete?
加工一批零件，甲单独做要12小时，乙单独做要10小时，丙单独做要15小时．现在先有乙、丙合作4小时，剩下的由甲单独来做，甲还需要几个小时才能完成？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) * 4</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 - (2/3)</gadget>
<output>1/3 = around 0.333333</output>

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

<gadget id="calculator">(1/3) / (1/12)</gadget>
<output>4</output>

<result>4</result>