To dig a canal, A completes the task in 4 hours alone, and B completes the task in 6 hours alone. Now A completes (1/3) first and then B alone does the rest. How many hours will it take to complete?
挖一条水渠，甲单独4小时完成任务，乙单独6小时完成任务．现在甲先完成(1/3)后余下的由乙独做，还需几小时才能完成？
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(2/3) / (1/6)</gadget>
<output>4</output>

<result>4</result>