This question already has an answer here:
Learning figures of speech sometimes can be confusing, and I am trying to figure out the difference between metonymy and synecdoche.
Given the following sentence
①Grey hairs should be respected (metonymy )
②More hands are needed in the work(synecdoche )
In Sentence 1, gray hairs is part of human body, so it should be included in synecdoche. So I cannot ensure that I'm right or my book represents a incorrect example.