Assume A + B = C, and assume A = 3 and B = 2.
Multiply both sides of the equation A + B = C by (A + B). We obtain A² + 2AB + B² = C(A + B)
Rearranging the terms we have A² + AB - AC = - AB - B² + BC
Factoring out (A + B - C), we have A(A + B - C) = - B(A + B - C)
Dividing both sides by (A + B - C), that is, dividing by zero, we get A = - B, or A + B = 0, which is evidently absurd.
I found this delouse paradox from http://www.paradoxes.co.uk