mathematical problem

(1) Four boys count as a whole, and the arrangement of ***5 elements is A55. The whole four boys have A44 kinds, so * * * has A55*A44 kinds.

(2) Arrange the other seven people first with A77, and then put Xiaoli in with C6 1, * * C6 1 * A77 = 6 * 7! =6*7*6! =A72*A66

(3) At this time, boys and girls passed the station. It can be divided into two situations: boys take the lead or girls take the lead. Let's assume that boys are the first. The arrangement of boys is A44, and that of girls is A44. At this time, there are A44*A44 species, and girls also have them, so there are 2 * * A44 * A44 species.

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