Question
Let $+_6 ($addition modulo $6) $ be a binary operation on $S = \{0, 1, 2, 3, 4, 5\}$. Write the value of $2 + _64^{−1}+ _63^{−1}.$

Answer

The composition table for $+_6$ on the set $S = \{0, 1, 2, 3, 4, 5\}$ is
$+_6$ $0$ $1$ $2$ $3$ $4$ $5$
$0$ $0$ $1$ $2$ $3$ $4$ $5$
$1$ $1$ $2$ $3$ $4$ $5$ $0$
$2$ $2$ $3$ $4$ $5$ $0$ $1$
$3$ $3$ $4$ $5$ $0$ $1$ $2$
$4$ $4$ $5$ $0$ $1$ $2$ $3$
$5$ $5$ $0$ $1$ $2$ $3$ $4$
$'0' $ is the identity element for $+_6$ from the table it is clear that
$4^{-1} = 2$ and $3^{-1} = 3$
Now, $2 + _64^{-1} + _63^{-1} = 2 + _62 + _63$
$= 4 + _63$
$= 1$

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