(3- 1)×4=6

6+ 1=7 (layer)

Xiaohong ran to the seventh floor.

Note: When Xiaoming ran to the fifth floor, he actually ran to the fourth floor. Because there was no need to run on the first floor, Xiaohong actually ran to the second floor when he ran to the third floor. When Xiao Ming ran to the 17 floor, that is, to the 16 floor, that is, the fourth floor, then Xiao Hong should have run to the fourth floor, that is, the sixth floor, plus the one he didn't have to run, that is, the seventh floor.