This is the 16044th most frequent Russian word.
пятнадцатую
"Пятнадцатую" means "fifteenth" in the accusative feminine singular form in English.
The word 'пятнадцатую' translates to 'fifteenth' in English, and is in the accusative case, feminine, singular form to agree with the direct object 'главу' (chapter).
Я читаю пятнадцатую главу этой книги.
I am reading the fifteenth chapter of this book.
The word 'пятнадцатую' is used in the accusative case, feminine, singular form agreeing with the direct object 'выставку' (exhibition), indicating sequence order.
Сегодня мы посетили пятнадцатую выставку по счету.
Today we visited the fifteenth exhibition according to count.
Here, 'пятнадцатую' is used as an ordinal number in the accusative case, feminine, singular form to describe the direct object 'гонку' (race).