Answer :
[tex]\begin{gathered} Length\text{ of AB} \\ \text{length = r}\cdot\theta \\ r=20\operatorname{cm} \\ \theta=120,but\text{ it must be in radians} \\ \theta\text{radians }=120\cdot\frac{\pi}{180} \\ \\ \theta\text{radians }=\frac{2\pi}{3} \\ \\ \text{length =(20)}\cdot(\frac{2\pi}{3}) \\ \\ \text{length}=\text{ 41.88}\approx42 \\ The\text{ length of the arc AB is 42cm} \end{gathered}[/tex]