A and B respectively process a batch of parts. A uses machine A to complete the task in 6 hours, and the efficiency of machine B decreases by 60%. B uses machine B to complete the task in 10 hours, and the efficiency of machine A increases by 20%. If A uses machine A and B starts to work at the same time, switches machines at a certain point in the middle, and finally finishes the task at the same time, find the time it takes for A and B to complete the task.
甲、乙两人分别加工一批零件，甲用A机器需要6小时才能完成任务，用B机器效率降低60%，乙用B机器需要10小时才能完成任务，用A机器效率提高20%．如果甲用A机器、乙用B机器同时开始工作，中途某一时刻交换机器，最后恰好同时完成任务，求甲、乙完成任务所用的时间．
<result>9</result>