$\begin{matrix}\frac{OA}{AC}=\frac{OB}{BD}  \\\frac{OC}{CE}=\frac{OB}{BD}  \\\end{matrix}\Rightarrow \frac{OA}{AC}=\frac{OC}{CE}\Rightarrow \frac{3/4}{3}=\frac{15/4}{CE}\Rightarrow CE=15$