To explain briefly,

In any case, people who choose computer principles do not need to calculate, because they choose another course, so only those who calculate discrete mathematics and data structures can, that is, calculate the total number of people MINUS those who study discrete mathematics and data structures.

25- 14- 12 but this will subtract two courses at the same time and add 6. This question is very confusing. Many conditions are actually mutually inclusive. I feel that the narrative can be clearer, otherwise it is difficult to consider.