Hola Inma,
You explain that we don't always need the articles though in certain phrases where we are not expressing a percentage of something but the lesson uses examples that all express percentages of something – where the ‘something’ is natural, eco-friendly, Spanish ancestry, and so on.
So can you please clarify to me why Cien por ciento de los vecinos apoyaron la decisión is incorrect and El 100 por cien de los vecinos apoyaron la decisión is correct?
I’m sorry but I have become quite confused with this particular lesson.