25. In the diagram, $A B / / E F / / D C$. Given that $A C+B D=250, B C=100$ and $E C+E D=150$, find $C F$.
25 Ans: 60 .
From the equalities $\frac{C E}{C F}=\frac{A E}{B F}=\frac{A C}{B C}$ and $\frac{D E}{C F}=\frac{B E}{B F}=\frac{B D}{B C}$, we get
$$
\frac{D E+C E}{C F}=\frac{B E+A E}{B F}=\frac{A C+B D}{B C} .
$$

Thus $\frac{150}{C F}=\frac{250}{100}$, and we have $C F=60$.