It takes 4 hours less time for A and B to complete a job together than B alone, and 9 hours longer for A to do it alone than A and B's cooperation time. How many hours does it take A and B to complete it together?
一件工作甲、乙合作的时间比乙单独完成时间少4小时，甲独做时间比甲、乙合作时间多9小时，甲、乙合作需几小时完成？
<gadget id="calculator">2 + 4</gadget>
<output>6</output>

<result>6</result>