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counting and probability intermediate
Problem
In how many ways can 7 people be seated in a row of chairs if two of the people, Wilma and Paul, refuse to sit next to each other?
Solution
The number of all seating arrangements is . The number of seating arrangements in which Wilma and Paul sit next to each other is . (We can arrive at by pretending Wilma and Paul together are one person, WilmaPaul, and that we have 6 chairs. We then have 6 people, who we can seat in ways. We then must break WilmaPaul back into two people, which we can do in ways, one for each order of the two -- Wilma then Paul, and Paul then Wilma. That gives us a total of ways to arrange the people with Wilma and Paul together.) Thus the number of acceptable arrangements is .
Final answer
3600