2. 外语
  3. Six people (two of them are John and Marry) are lined in a row. How many arrangements are there if John and Marry are at both en

Six people (two of them are John and Marry) are lined in a row. How many arrangements are there if John and Marry are at both en

admin2022-11-20  69
问题 Six people (two of them are John and Marry) are lined in a row. How many arrangements are there if John and Marry are at both ends of the row?
选项 A、720
B、30
C、48
D、2
E、24
答案C
解析 先考虑有限制的元素，J和M在两头，那么恰好是在两个位置上的全排列，为P22=2种，剩下的4个人在中间4个位置上全排列，为P44=24种，所以J和M在两头的排列一共有24×2=48种。
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