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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:

(a) 3 and 13

(b) 2 and 11

(c) 4 and 13

(d) 8 and 14
edited | 775 views

Option c.

Identity element here is 1.

4 * 4 mod 15 = 1

7 * 13 mod 15 = 1.
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+7

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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