The given problem is an example of an "inscribed angle"
The inscribed angle ∠ACB is half of the intercepted arc AB
[tex]\angle ACB=\frac{1}{2}\text{mAB}[/tex]
The intercepted arc AB is 40°
So, the inscribed angle ∠ACB becomes
[tex]\begin{gathered} \angle ACB=\frac{1}{2}(40\degree) \\ \angle ACB=20\degree \end{gathered}[/tex]
Therefore, the measure of ∠ACB is 20°
