Nine chairs are in a straight line and numbered $$1$$ to $$9$$ from left to right. Five girls and four boys sit in the chairs so that no girl is next to another girl. A boy could be sitting in the chair with which number? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
The girls must be in the odd-numbered seats; otherwise, two girls would be seated next to each other. This leaves only the even-numbered seats for the boys, so a boy is seated in seat $$4$$. 