First off, I'm sorry if this is a simple question and I just don't know geometry.
The question I'm wondering about is from the 2018-19 Meet 4, individual event B for math team:
In $Figure$ $2,$ a circle of radius $20$ contains three points, $A$, $B$, and $C$. Two chords, $\overline{AB}$ and $\overline{BC}$, are drawn. If the length of $\newcommand{arc}[1]{\stackrel{\Large\frown}{#1}}\arc{BC}$ is $\dfrac{5\pi}{3}$, determine exactly the measure of $\angle{BAC}$.
The solution that they have stated is $\dfrac{\pi}{24}$. Their explanation is: "Let x = the central angle intercepting $\newcommand{arc}[1]{\stackrel{\Large\frown}{#1}}\arc{BC}$. Then $\dfrac{\frac{5\pi}{3}}{40\pi}=\dfrac{x}{2\pi} \rightarrow x=\dfrac{\pi}{12}$. Then $m\angle{BAC}=\frac{\pi}{24}.$" I understand how they solve it once they have $\dfrac{\frac{5\pi}{3}}{40\pi}=\dfrac{x}{2\pi}$, but where do they get those numbers to start with? Is there a specific theorem that I don't have written down on my cheat sheet or it just something simple? Thanks in advance!