one-fifth vs twenty percent

one-fifth and twenty percent both are nouns.

Word	Noun	Adjective	Verb	Adverb
one-fifth	Yes	No	No	No
twenty percent	Yes	No	No	No

As nouns, one-fifth and twenty percent are synonyms defined as:

Other synonyms of one-fifth include fifth, fifth part.

one-fifth (noun)	twenty percent (noun)
one part in five equal parts	one part in five equal parts