Answer:
m = 7, n = 5
Explanation:
3m+3n=36
m + n = 12
n = 12 - m
8m-5n=31
8m - 5(12 - m) = 31
8m - 60 + 5m = 31
13m = 91
m = 7
n = 12 - 7
n = 5
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