fifth part vs twenty percent

fifth part and twenty percent both are nouns.

Word	Noun	Adjective	Verb	Adverb
fifth part	Yes	No	No	No
twenty percent	Yes	No	No	No

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

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

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