15. The school offers four extracurricular interest classes in Chinese, Math, Foreign Language, and Natural Science for students to voluntarily sign up for. The number of students who want to participate in the Chinese, Math, Foreign Language, and Natural Science interest classes are 18, 20, 21, and 19, respectively. If the total number of students in the class is 25, how many students at least have signed up for all four interest classes?
Three, 15. The number of people not attending the Chinese interest class is 7, the number of people not attending the Math interest class is 5, the number of people not attending the Foreign Language interest class is 4, and the number of people not attending the Natural Science interest class is 6. Therefore, the maximum number of people who do not attend at least one interest class is 22, which means the minimum number of people who attend all four interest classes is 3.

Construct a scenario where exactly 3 people attend all four interest classes as follows:

3 people attend all four interest classes, 7 people attend the three interest classes except for Chinese, 5 people attend the three interest classes except for Math, 4 people attend the three interest classes except for Foreign Language, and 6 people attend the three interest classes except for Natural Science.