twenty dollar bill vs twenty

twenty dollar bill and twenty both are nouns.

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

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

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