某项工程由甲队单独做需18天完成，由乙队单独做只需甲队的一半时间完成．设两队合作需x天完成，则可得方程（   ）

    My name's John．Today is my birthday．I'm nine．I like animals(动物)．I like pandas．They are funny．I like eating, too．I like fish and eggs．I have a birthday cake and many gifts．Let's eat the birthday cake，OK?

一组数据2，3，x，5，7的平均数是4，则这组数据的众数是____