Given AD || EF, calculate FD/CF

We are given that $AD \parallel EF$. Since $G$ and $F$ are points on $EF$, it means $AD \parallel GF$.
Consider $\triangle CAD$. The line segment $GF$ is parallel to $AD$.

Since $GF \parallel AD$ in $\triangle CAD$, by the Basic Proportionality Theorem, the line $GF$ divides the sides $AC$ and $CD$ proportionally.