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The set $\{1, 2, 4, 7, 8, 11, 13, 14\}$ is a group under multiplication modulo $15$. The inverses of $4$ and $7$ are respectively:

1. $3$ and $13$
2. $2$ and $11$
3. $4$ and $13$
4. $8$ and $14$
edited | 841 views

Option C.

Identity element here is $1.$
$4 * 4 \mod 15 = 1$
$7 * 13 \mod 15 = 1.$
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It is same as finding modular inverse, which is also last step in RSA.

You may likely to know actual procedure for even big numbers-

https://goo.gl/ZCza9r

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