GRE triangle area question

Question

I dont understand why AE is 1 if AD is 4 and the ratio between CD and AB is 9/3 or 3

Answer 1

Since $AE:ED=1:3$ by similar triangles hence $3AE=ED$. Notice that:

$4=AD=AE+ED=AE+3AE=4AE$ hence $AE=1$.

The ratio between $CD$ and $AB$ is 3 since $\frac{CD}{AB}=\frac{9}{3}=3$.