AllHowever, I have a bit of difficulty prove all elements are units.non-zeroelements are units. First the identity element is 6. Next for every non-zero element, say 2, we need to show it has an inverse. That is, 2*x = 6 (because 6 is unity). So what does 2 multiply with to give 6 under mod 10. Then do this for the others.