72. In how many ways can 10 athletes be divided into two teams of 5 people each, if two athletes wish to play on different teams?
72. If we temporarily separate two athletes who refused to play on the same team, 8 athletes will remain. Let's add four more athletes to one of the previously separated athletes. This can be done in $C_{8}^{4}=70$ ways. The remaining 4 athletes will join the team of the second athlete. A n s w e r. 70.