To produce a batch of parts, A alone takes 15 hours, B alone takes 12 hours, and C takes 10 hours. A and B work together for 3 hours, and then C does it alone. How many hours will it take to complete?
生产一批零件，甲单独完成需要15小时，乙单独完成需要12小时，丙要10小时．甲乙先合作3小时后，再由丙单独做，还要几小时才能完成？
<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">(1/15) + (1/12)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/20) * 3</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">1 - (9/20)</gadget>
<output>11/20 = around 0.55</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(11/20) / (1/10)</gadget>
<output>11/2 = around 5.5</output>

<result>11/2 = around 5.5</result>