Assuming you mean baseball in stead of basketball, if must be the 5th floor.
Step-by-step explanation:
"The girl on the fifth floor thinks soccer is boring" implies that on the 5th floor lives the football or baseball fan. However, it cannot be the football fan since it is already known that she lives on 6th.
She only asked the students in her ballet class, which is probably biased toward ballet. She should survey everyone in The Dance Academy. That way, the answers won't be biased.
