In all honesty, this is a bit of a dumb question. So I recently had a conversation about a video game and my friend said "99.9% of the League community is degenerate." I told him that it's actually "are degenerates" but he insisted that he was correct because it is an adjective whilst I know that it is an adjective, "is degenerate" sounds wrong, and fairly certain it is. So I'm asking if "99.9% of the League community is degenerate." is grammatically correct. Specifically the "is degenerate" part.
Here's how I see it: "99.9% of the League community" is a noun phrase. The nucleus of this phrase is the number "99%", which is referring to a portion of the "community", which is a collective noun. That means it is a bit ambiguous whether it should be treated as singular (that part of the community is a single entity) or plural (those who are in that part of the community are making each other look bad).
In either case, the verb "to be" can either be predicative (introducing a description) or copulative (like an equals sign).
If "to be" is being used in the predicative sense, then "degenerate" is an adjective describing the community. The sentence can read one of two ways:
- "99.9% of the League community is degenerate." (where 'community' is a singular collective noun)
- "99.9% of the League community are degenerate." (where 'community' is a plural noun)*
If on the other hand "to be" is being used in the copulative sense, then the community is being defined as "degenerates", which is being used as a noun. In this usage, it can only refer to an individual person, not a group. You can't say "the community is a degenerate". So you would be forced to say:
- "99.9% of the League community are degenerates." (plural noun)
Depending on your intended meaning, any of the three bullet points listed above could be correct.
I do also have a general preference for percentages being treated as singular, ("5% is unemployed", rather than "5% are unemployed"). If you share my preference, then that would eliminate the second bullet point (*) above, leaving only "is degenerate" and "are degenerates" as the only two grammatically correct options.