Mathematics problems and answers in senior two.

1, the empty seats are not adjacent, that is, there are 7 empty seats among 6 people, and four empty seats are arranged from them, that is, C74 (this is the arrangement formula, and the correct spelling is 7 below and 4 above, I don't know how to type, hehe) =7*6*5/3*2* 1=35.

2. Only three of the four vacancies are adjacent, that is, C72=7*6/2=2 1.

3. Classification: It can be divided into three possibilities: non-adjacent, two adjacent and three adjacent, namely, c74+c73+c72 = 35+35+21= 91.

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