To determine the additive inverse of 15 mod 7:
Write down -15, i.e., the opposite of the number whose modulo you need.
Repeatedly add 7 to -15. In this way, we obtain the sequence -15, -8, -1, 6, 13, 20...
From this sequence, we pick the number between 0 and 6. It is 6.
We have found the additive inverse of 15 mod 7!
Read more