The spring sports meeting is coming soon, and every student in Class 1 of Grade 6 has signed up for at least one event. If 40 students are participating in rope skipping and 31 students participating in the long jump, and 21 students participating in both events, how many students are participating in only one event? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets