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 there are 40 students participating in skipping rope and 31 students participating in 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