This is the 11242nd most frequent Russian word.
двадцатой
"Twentieth" (ordinal number).
"Двадцатую" is the accusative feminine singular form in this context, matching "главу."
Сегодня я закончил двадцатую главу этой книги.
Today, I finished reading the twentieth chapter of this book.
"Двадцатой" is the prepositional feminine singular form in this context, matching "странице."
На двадцатой странице я нашел интересную фотографию.
On the twentieth page, I found an interesting photo.
"Двадцатой" is the genitive feminine singular form in this context, matching "годовщины."