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Algebra Level 5

$\large f(20 + x) = f(20 - x)$

Let $f:\mathbb{R} \to \mathbb{R}$ such that $\forall x \in \mathbb{R}$ , the above is true. Given that $f$ has exactly 3 real roots at $a$ , $b$ and $c$ , find the value of $a+b+c$ .