Maths behind ZKPs

What is -13 mod 5?

Consider the set of Integers S = {0,1,2,3,4,5,6}. What does 4 + 4 equal?

Consider the set of Integers S = {0,1,2,3,4,5,6}. What does 3 x 5 equal?

Consider the set of Integers S = {0,1,2,3,4,5,6}. What is the inverse of 3?

For S = {0,1,2,3,4,5,6} Can we consider ‘S’ and the operation ‘+’ a group?