Calculating possible option of switching pairs in group I am struggling with this probability/combinatorics problem.
the problem states: "11 people are seated around a round table. At some point, everyone stands up and sits again at a randomly chosen seat. Show that the expected value of the number of pairs which swapped places (A sat at B's place and B sat at A's place) is 1/2."
I struggle with calculating the number of orderings in which pairs switch places. I know we have 10! possible permutations for 11 people sitting in a circle, and there are $11\choose2$ possible pairs that could switch places. But I'm not sure how to count for example the number of permutation in which only one couple switches places.
 A: Number the people from 1 to 11 and define
$$X_{A,B} = \begin{cases}
1 \quad \text{if A and B switch places} \\
0 \quad \text{otherwise}
\end{cases}$$
for $1 \le A < B \le 11$. 
The probability that $A$ ends up in $B$'s former position is $1/11$, and then the probability that $B$ also ends up in $A$'s former position is $1/10$; so
$$P(X_{A,B} = 1) = \frac {1} {11 \cdot 10}$$
The expected number of pairs who switch places is 
$$\begin{align} E \left( \sum_{1 \le A < B \le 11} X_{A,B} \right) &=
 \sum_{1 \le A < B \le 11} E(X_{A,B} ) \tag{1}\\
&= \binom{11}{2} \frac {1} {11 \cdot 10} \tag{2}\\
&= \frac{1}{2}
\end{align}$$
At $(1)$ we have used linearity of expectation, and at $(2)$ we have used the fact that the number of pairs $(A,B)$ satisfying $1 \le A < B \le 11$ is $\binom{11}{2}$, the number of subsets of $\{1,2,3,\dots,11\}$ of size $2$.
A: There's a total of $11!$ possible permutations. A given pair $(a,b)$ gets switched in exactly $(11-2)! = 9!$ of these: the elements $a$ and $b$ get swapped while the rest (the other 9 elements) are free to do whatever (to permute among themselves). There is a total of $\frac{11*10}{2}$ pairs $(a,b)$, giving a total of $9!\frac{11*10}{2}$ switchings for all pairs in all permutations. Dividing by the number of permutations gives an average of $\frac{1}{2}$ switchings per permutation.   
