Arithmetic modulo n: a ≡ b (mod n) means n divides (a−b), i.e. a and b have the same remainder when divided by n. Congruence is reflexive, symmetric, transitive; compatible with +, −, ×.. The key formula is a ≡ b (mod n) ⟺ n | (a−b). This concept is typically introduced in College. Understanding this concept builds a strong foundation for more advanced mathematics.