The inverse of the multiplication, module 5 is: the two values are multiplied with modul 5. The inverse is the rest of the division when the division it's iqual to 1, because 1 it's a neutral element of the multiplication. Example: 3*2 = 6 and 6 mod 5 = 1 because it's the rest. Result: The inverse of the 2 it's 3 and the inverse of the 3 it's 2. Zero has no inverse because it is not an element of the list.