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10 students a...

Question

10 students are sitting in a circle , Find the number of ways to select two non-consecutive students of them.

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Solution

No. of ways of selecting first student = 10
No. of ways of selecting second student = 7 (except first student and his neighbours )
But every time ther pair (AB) comes pair (BA) also comes .
So final answer is $\frac{10 \times 7}{2} = 35$

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