Since a unified GDZ website for this specific author is rare, if you are stuck on a particular problem:
: You can download digital copies (DJVU/PDF) of the problem book, which include these back-of-the-book answers, from specialized archives like StatPhys or the Cosmatica Library . gdz na zadachnik v.p.demkovich, l.p.demkovich
However, you can find the problems and their respective answers through the following resources: Available Resources for Demkovich Solutions Since a unified GDZ website for this specific
ДЛ9. Демкович. Сборник задач по физике which include these back-of-the-book answers
: Platforms like VK (Soviet Textbooks group) often share scans and manual solutions for classic Soviet-era problem books.