[ Diameter, main properties $\quad]$ [ Ratios of areas of similar figures ]

On the side $B C$ of triangle $A B C$ as a diameter, a circle is constructed, intersecting the segment $A B$ at point $D$. Find the ratio of the areas of triangles $A B C$ and $B C D$, given that $A C=15, B C=20$ and $\angle A B C=\angle$ $A C D$.