Using adjectives as nouns
Solution 1:
Grammatically, Quirk, Greenbaum, Leech and Svartvik (1985) describe this as adjectives functioning as heads of noun phrases' (7.23—7.26, pp 421—424).
Rhetorically, this is a combination of ellipsis (the omission of a word or phrase which is easily understood in context) and metonymy (referring to something or someone by one of its/their attributes).