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Let q subset N x N such that q(x, y) <=> x mod 5 = y mod 5. We have
[0]q = {0, 5, 10, 15, 20, 25, ...}, [1]q = {1, 6, 11, 16, 21, 26, ...}, [2]q = {2, 7, 12, 17, 22, 27, ...}, [3]q = {3, 8, 13, 18, 23, 28, ...}, [4]q = {4, 9, 14, 19, 24, 29, ...}, [5]q = {0, 5, 10, 15, 20, 25, ...}, ...
We see that [0]q union [1]q union [2]q union [3]q union [4]q = N and that any two of these sets are disjoint.