twenty vs twenty dollar bill

twenty and twenty dollar bill both are nouns.

Word	Noun	Adjective	Verb	Adverb
twenty	Yes	Yes	No	No
twenty dollar bill	Yes	No	No	No

As nouns, twenty and twenty dollar bill are synonyms defined as:

twenty (noun)	twenty dollar bill (noun)
a United States bill worth 20 dollars	a United States bill worth 20 dollars
the cardinal number that is the sum of nineteen and one

twenty (adjective)	twenty dollar bill (adjective)
denoting a quantity consisting of 20 items or units