1. The problem statement, all variables and given/known data 2. Relevant equations a. I know that x*a mod y should be the same as y*b mod x but I don't understand why b. I know that an inverse can be constructed because x and y are mutually prime and gcd(x,y) = 1 , but I have no clue at what pair x and m is possible c. I have no idea how to do c.) 3. The attempt at a solution I look at the notes from lecture, and I look at some discrete math text book (we don't have a text book) but still have no clue, I also search the web regarding modular arithmitic. Any hint regarding this subject would be really appreciated.