2.5
1) Since we have similar triangles, then those triangles have congruent angles and proportional sides.
2) So we can write the following proportion:
[tex]\begin{gathered} \frac{AB}{DE}=\frac{AC}{DF} \\ \frac{2}{1.25}=\frac{4}{DF} \\ \text{Cross multiply them} \\ 2DF=4\times1.25 \\ DF=\frac{5}{2} \\ DF\text{ = 2.5} \end{gathered}[/tex]
3) Hence, DF has a length of 2.5 units
