Below (after the reminder) is the answer to the Problem 4 of the lesson Some logic problems (the topic Miscellaneous word problems
of the section Word problems in this site).
Problem 4 formulation (reminder)
There are 25 students in the class.
Each student attends a sport class or an art school. Some of students attend both a sport class and an art school.
Number of the students attending a sport class is 18; number of the students attending an art school is 12.
How many of class students attend both a sport class and an art school?
The number of students attending both a sport class and an art school is equal to 5.