15. Find the unknown angle

Let us consider the image given below and answer the question that follows.

 

 

Here, for a circle with $O$ as the centre, area of the sector $OAPB$ is $\frac{9}{36}$ times the area of the circle. Find the angle subtended by '$O$' to form the sector.

 

The angle subtended by '$O$' to form the sector $=$ $^\circ$