#### Answer

$DE = 7.5$

#### Work Step by Step

The diagonals of a parallelogram bisect one another; therefore, if the entire diagonal is $15$, then each bisected segment is half of this length:
$DE = \frac{1}{2}(15)$
Multiply to simplify:
$DE = \frac{15}{2}$
Divide to solve:
$DE = 7.5$