Good, adjective, = buono. Well, adverb, = bene. I am a good writer. I write well.
The problem is that in colloquial English we (British and American) have come to use "well" as an adjective when referring to ones personal condition. It's probably best to see that as an aberration.
In Italian, stanno bene (translates as they are well) is actually using bene as an adverb - they stand well. I think :-)
Using "well" as an adjective referring to health is not a colloquialism; it is good English in Britain and America. "I am well," means "I am in good health. Ex: "Are you well?" "He is well now." When someone asks you how you are and you say "I am well" you are saying "I am in good health," and "well" is a predicate adjective; it is not short for "I am doing well."
Yes, Duolingo, "they are very good" is grammatically incorrect. Yes, we should be saying "very well." It's incredibly pedantic to not accept "very good" as a translation, however, because I think you'll find that 90% of the time a native speaker will say good instead of well when speaking to you.
someone just mailed me to say that "one hundred of them" definitely takes the plural. As an English teacher here, it depends on whether "one hundred" is seen as a unit or not. If unit, singular; if not, plural. Also doesn't help that we don't know what "loro" stands for, people or things.