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:
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
Gatecse