A job is completed in 15 hours alone, and B is completed in 12 hours alone. Now B works alone for 3 hours, and then A and B work together. How many hours will it take to complete?
工作，甲独做15小时完成，乙独做12小时完成，现乙先单独做3小时，然后甲、乙两人合做，还需几小时才能完成？
<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 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) + (1/12)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/4) / (3/20)</gadget>
<output>5</output>

<result>5</result>