Teo, Can you explain this again, please, with more details? The blanks are placed where prepositions are not used, so I feel the exercise is wrong, or incorrectly copied, or something. Besides, the sentences seem to have nothing to do with "in/on the fifth floor". (It's "on the fifth floor", by the way.) Jim
As I jumped off the building... I saw the known loving couple in 10F is hitting each other. I saw the usually tough & strong Peter in 9F is crying. 8F Ah Mei just found out her fiancee is sleeping with her best friend. 7F Dan is taking his daily anti-depression medicine. 6F jobless Heng still buys 7 newspaper to search for a job every day. Much respected Mr. Wong in 5F is trying his w
Okay, in the above paragraph the designations 10F, 9F, etc refer to room numbers not to the floor they are on. So it is correct to say Mr. so and so in (room) 10F.
If the writer wants to refer to the floor, he should say Mr. Wong on the 5th floor as CJ mentions above.