Right. The best thing you can do is say the numbers are an element of the set of tuples {(x, 6-x) : x \in R}. And that's only if you restrict to reals. Consider

Right. The best thing you can do is say the numbers are an element of the set of tuples {(x, 6-x) : x \in R}. And that's only if you restrict to reals. Consider the fact that the "average" of i and 6-i is also 3. In general, you replace R by k, where k is your field (or even any unital ring where 2 = 1+1 is invertible). Thus, in the finite field F_5, the possible numbers are (0,1), (1,0), (2,4), (4,2), (3,3). More cleanly, you can take the k-points on the variety x + y - 6 = 0.