A line through the point D on side AB of triangle ABC runs parallel to BC and meets $AC$ at $E$. Only two lengths are recorded: AD = 4.8 cm and AB = 12 cm. Find $\frac{AE}{EC}$.

The question asks for a part-to-part ratio, but $AB$ is a whole side, so first recover $DB=AB-AD=12-4.8=7.2$ cm. Then $\frac{AE}{EC}=\frac{AD}{DB}=\frac{4.8}{7.2}=\frac{2}{3}$. The option $\frac{2}{5}$ is $\frac{AD}{AB}$ — answering with part over whole, the standard BPT error; $\frac{3}{2}$ is the correct ratio inverted; $\frac{5}{2}$ is $\frac{AB}{AD}$, whole over part.