Question 61
$A = \{a, b, c\}$ is a set containing three positive integers. Prove that we can find a set $B \subset A$, $B = \{x, y\}$ such that for all odd positive integers $m, n$ we have
$$
10 \mid x^m y^n - x^n y^m
$$
$$
10 \mid x^m y^n - x^n y^m
$$