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This equation only tells us that A = C but doesn't tell us the value of either, so "A + C" could be "2A" or "2C" but they could be anything.

10 says my sleepy mind. Half is always the best conservative approach to go for lol.

A+C = 2x where x can be any number between 0 and 10 if we use only positive numbers for B.

as per calculation a= c , then 2c=10. then c = 5. so a+c -= 10