Question 27
$ABC$ is a triangle with $AC = BC$ and $\angle ACB \lt 60^{\circ}$. Let $A'$ and $B'$ be two points on the sides $BC$ and $AC$ respectively such that $AA' = BB' = AB$. Let $C'$ be the intersection of $AA'$ with $BB'$. Knowing that $AC' = AB'$ and $BC' = BA'$, what is the magnitude of the angle $\angle ACB$ in degrees?