Let Z 5 be the relation of congruence modulo5.

If 1 ≤ m, n ≤ 5,we define the product of two classes as [m][n] = [mn].
Explain why [3][4] = [2].

I know 3 x 4 = 12. 5 mod 12 = 2.
but i don't know how to explain it.

Help with the explanation please.
Thanks