MathNet 2015-part-06 — 191 problems (EN–VI) Question 58

The circle centre $O$ has radius $4$ cm. If the area of sector $OPQ$ is $\frac{1}{2}\pi$, and if the probability that a point chosen randomly in the circle lies in the shaded sector is $\frac{1}{n}$, then the value of $n$ is