Example 2. The area of equilateral $\triangle ABC$ is $36, P, Q, R$ are points on $BC, AB, CA$ respectively, and $BP=\frac{1}{3} BC, AQ=QB, PR \perp$ $AC$, then the area of $\triangle PQR$ is ( ).
(A) 10
(B) 12
(C) 14
(D) 15
(1988, 4th National Partial Provinces, Municipal Junior High School Correspondence Competition)
Yi As shown in Figure 3, from the conditions, it is easy to know that $\frac{A Q}{Q B}=1, \frac{B P}{P C}=$ $\frac{1}{2}, \frac{C R}{R A}=\frac{1}{2}$. Given $m=1$, $n=\frac{1}{2}$, $l=\frac{1}{2}$, we can find that $S_{\triangle P Q R}=10$. Therefore, the correct choice is (A).