7・ 1 There are 9 chairs around a round table, and 4 people sit down randomly. What is the probability that no two people are sitting next to each other?
Solution
[Solution] The first person can sit in any position, and the remaining three people have a total of ways to sit in a circle.
On the other hand, if no two people are to sit next to each other, the first person can sit in any position, and the remaining 3 people can sit in alternating seats in 6 ways, and the extra chair can be in 4 positions, so the total number of ways is .
Therefore, the required probability is .
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