Do 20 pages of a book gives you 90% of its words?

My English teacher claimed, that if I survive the first 20 pages of any book, the reading will get much easier because the words that occurred on those pages constitute 90% of all words in the book. So after you reach this threshold you don’t have to go back and forth to the dictionary.
Was he right? (TLDR: Kind of.)

From Do 20 pages of a book gives you 90% of its words? – Vocapouch