Angles · Superchallenge problem

Four rays $OA$, $OB$, $OC$, $OD$ come out of point $O$ and split the full turn around $O$ into four angles: $AOB$, $BOC$, $COD$ and $DOA$. Angle $AOB$ is $20^\circ$ bigger than angle $BOC$, angle $BOC$ is $20^\circ$ bigger than angle $COD$, and angle $COD$ is $20^\circ$ bigger than angle $DOA$. How many degrees is angle $AOB$?