KöMaL 2016 C1383

The midpoint of line segment $AB$ is $O$, the point closer to $A$ dividing $AB$ into a ratio $1:4$ is $C$, and the point closer to $B$ dividing $AB$ into a ratio $1:4$ is $D$. The perpendiculars drawn at points $C$, $O$ and $D$ to the line segment $AB$ intersect the semicircle of diameter $AB$ at the points $P$, $Q$, $R$, respectively. Determine the angles of triangles $PQB$ and $QRB$.