I3.3 In the figure, $A P / / C R / / B Q, A C=x, C B=12, A P=10$, $B Q=15$ and $C R=y$. Find $y$.
$\begin{array}{l}\text { Draw } A D E / / P R Q \text {, cutting } C R \text { at } D, B Q \text { at } E \text { respectively. } \\ A P R D, A P Q E \text { are //-grams } \\ D R=10=E Q \text { (opp. sides, //-gram) } \\ C D=y-10, B E=15-10=5 \\ \triangle A C D \sim \triangle A B E \text { (equiangular) } \\ (y-10): 12=5: 24 \text { (ratio of sides, } \sim \Delta \text { 's) } \\ y=12.5\end{array}$