Answer:
Length of DE is 25.
Explanation:
Right, so this is something called midsegment theorem. ΔDBE is similar (but not congruent) to ΔABC because of the angles they share. Since DE is described as a midsegment, this means BE=EC and BD=DA; so ΔDBE is half the size of ΔABC, meaning its side lengths are also half that of the larger triangle.
(Sorry if this information was unnecessary. Just got an answer deleted because I wasn't detailed enough.)