It takes 6 hours for A to do a job alone, and 4 hours for B to do it alone. After A has done (1/6) of the job, how many hours will it take for both of them to cooperate?
一件工作，甲单独做需要6小时，乙单独做需要4小时，甲做了这件工作的(1/6)后，两人合作，还需几小时才能完成？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 - (1/6)</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/6) + (1/4)</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">(5/6) / (5/12)</gadget>
<output>2</output>

<result>2</result>