Hello. I have given the thread a new purpose, seeing as how it was a double post.

Find the complete set of residues modulo 10 whose elements are

a) nonnegative

b) odd

c) even

a) {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

b) and c) are challenging me though. I have a feeling it is going to be too simple. If the modulo was any odd number, I could do this question in a heart beat (and I am not saying that is any great feat). But seeing how 10 is even, I cannot do the ol' simple a+p (modp) = a method. What can I do? What should I look for? Is it something to do with Euler's function and negative numbers?