Since triangle ABC and triangle DEF are similar triangles, the length of DF is equal to 3.75 units.
In Mathematics and Eucldiean Geometry, two triangles are said to be similar when the ratio of their corresponding side lengths are equal and their corresponding angles are congruent.
Additionally, the lengths of corresponding sides or corresponding side lengths are proportional to the lengths of corresponding sides when two (2) triangles are similar.
15/DF = 24/6
By cross-multiplting, we have;
24DF = 90
DF = 90/24
DF = 3.75 units.