It's actually not quite as clear cut as rewriting 25 percent. There is actually no percentage that clearly makes content duplicate or unique. Search engines like google use shingling to determine if an article is unique or not. In this process they group small phrases together then compare each phrase to all other phrases in their database. The overall page score determines the uniqueness of the article. I wrote a fairly detailed blog post on the subject if anyone wants to read more about it you may do so here.
- Simplified SEC
In the end I wouldn't reuse articles provided by anyone else unless you plan to change the majority of the text in some way.