Answer:
DE = 5
Step-by-step explanation:
The 3 sides of the triangle are congruent , then
DF = EF , that is
[tex]\frac{2}{5}[/tex] a - 15 = [tex]\frac{1}{5}[/tex] a - 5 ( multiply through by 5 to clear the fractions )
2a - 75 = a - 25 ( subtract a from both sides )
a - 75 = - 25 ( add 75 to both sides )
a = 50
Then
DE = EF = [tex]\frac{1}{5}[/tex] × 50 - 5 = 10 - 5 = 5
