.
Then the number of students of those 100 surveyed who took at least one course was
70 + 45 + 60 - (30+38+36) + 23 = 94,
and the number of those who took NO ONE of these courses was 100-94 = 6.
On this subject see the lessons
- Counting elements in sub-sets of a given finite set
- Advanced problems on counting elements in sub-sets of a given finite set
in this site.
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It is a beautiful problem, and I like to explain it to students in all details, but this time I was partly upset
by not seeing a question in your post . . .