b is the inverse of a $( \mod 11)$

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Let a and b be numbers in the set $S = \{0, 1, 2, 3, 4, 5, 6, 7, 8 , 9, 10\}$ such that b is the inverse of a $(\mod11)$ and a and b are not equal. How many such subsets $ \{a, b\}$ of S are there?

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Since $11$ is prime, every $a\in\mathbb{Z}/11\mathbb{Z}$ has an inverse except for 0. We can calculate a list of inverses and note that $1\cdot1=(-1)\cdot(-1)=1$ and that no other number is it's own inverse, leaving us with $8$ numbers. Each pair such that $a\cdot b = 1$ gives us one set, sinec $\{a,b\}=\{b,a\}$, and so we divide by 2 to get $4$ total sets.

They are: $\{2,6\},\{3,4\},\{5,9\},\{7,8\}$