7. In the figure below, $ABCD$ is a trapezoid, $AD // BC$, $\frac{BC}{AD}=5$, and the area of $\triangle OAD$ is $S$.
(1) Express the area of $\triangle OBC$ in terms of $S$;
(2) How many times the area of trapezoid $ABCD$ is the area of $\triangle OAD$?
7. (1) $25 S$; (2) 36 times.