A project can be completed by A alone in 10 hours, and by B in 12 hours. After 3 hours of cooperation, A alone can complete the rest. How many hours will A need to complete?
一项工程，甲单独做10小时完成，乙单独做12小时完成，两人合作3小时后，剩下的由甲单独完成，甲需要几小时才能完工？
<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) * 3</gadget>
<output>11/20 = around 0.55</output>

<gadget id="calculator">1 - (11/20)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(9/20) / (1/10)</gadget>
<output>9/2 = around 4.5</output>

<result>9/2 = around 4.5</result>