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    精英家教网 > 初中数学 > 题目详情
    There is a piece of work．It takes Mr．A alone 20 days to finish，and Mr．B 30 alone days to finish．It takes them（　　）days to work together to finish the work．
    A．10B．12C．15D．50
    设他们一起工作x天可结束工作．
    （
    1
    20
    +
    1
    30
    ）x=1
    x=12
    需要12天可完成工作．
    故选B．
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