Answer:
4 units
Step-by-step explanation:
In triangle DEF, DK is the angle bisector of angle EDF.
Angle bisector theorem states that if a ray bisects an angle of a triangle, then it divides the opposite side into segments that are proportional to the other two sides.
Hence,
[tex]\dfrac{DE}{DF}=\dfrac{EK}{KF}[/tex]
Thus,
[tex]\dfrac{DE}{10}=\dfrac{2}{5}\\ \\5DE=2\cdot 10\\ \\5DE=20\\ \\DE=4[/tex]
