Use the ordinal numbers.
A.This is flat.This is the .... flat
B.Mary lives on floor eleven.Mary lives on the ... floor.
C.Book three is about traveling.The ... book is about traveling СРОЧНО НАДО!ПОЖАЛУЙСТА!!!!!

A. This is flat 5.  This is the fifth  flatB. Mary lives on floor eleven.    Mary lives on the eleventh floor.C. Book three is about travelling.   The third book is about travelling