Why do we need "an" in these sentences below?
I run an additional three-tenths of a kilometer.
I write an additional 5 words.
As an English learner, I would omit "an," because it's illogical for me. I see plural forms, so my first reaction is to omit the articles. I would say:
I run additional three-tenths of a kilometer.
I write additional 5 words.
Please explain me the differences between these sentences, because my all grammar knowledge has just fallen.
I found a semi-explanation here, does it make any sense?