Answer :
[tex]\bf \textit{arc's length}\\\\
s=r\theta ~~
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad radians\\
------\\
r=10\\
\theta =\frac{\pi }{3}
\end{cases}\implies s=10\cdot \cfrac{\pi }{3}\implies s=\cfrac{10\pi }{3}[/tex]