A batch of parts is processed by one person alone. A takes 12 hours and B takes 10 hours. Now A will do 3 hours first, and B will join in to do it together. How many hours will it take to complete?
加工一批零件，由一个人单独做，甲要12小时，乙要10小时，现在甲先做3小时后，乙也来参加一同做，还需多少小时完成？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

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

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

<gadget id="calculator">(3/4) / (11/60)</gadget>
<output>45/11 = around 4.090909</output>

<result>45/11 = around 4.090909</result>