Two Triangles in a Decagon

What fraction of this regular decagon is shaded?

Let S be the area of the regular decagon.

Connect points C and F. Since AB, DE, and is parallel to CF from symmetry, we get that S(ABC) = S(ABO) = S(DEO) = S(DEF) = S/10.

Therefore, 80\% of the total area is shaded.