12. (11 points) Class 3 (1) has a total of 45 students who have signed up for the school sports meet. Apart from the tug-of-war competition, in which the entire class participates, each student must participate in at least one of the other three events. It is known that 39 students have signed up for the shuttlecock kicking competition, and 28 students have signed up for the basketball shooting competition. How many students have signed up for all three events?
【Answer】Solution: According to the problem, all students in the class participate in the tug-of-war competition, so we only need to consider the number of students who sign up for all three competitions, which is the number of students who participate in both kicking the shuttlecock and basketball shooting. By the principle of inclusion-exclusion, the number of students is $39+28-45=22$. Answer: 22 students have signed up for all three competitions.