.
The answer is 85 + 50 - 120 = 135 - 120 = 15.
Explanation
I do not need to explain why I added 85 and 50 - it is clear.
I need only to explain why I subtracted 120.
It is because when I added 85 and 50, I exceeded the value of 120.
I exceeded it exactly by the value of those who study both languages.
Therefore, when I subtracted 120, I exactly got the number of those who study both languages.
Answered, solved, explained and completed.
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See the lesson
- Counting elements in sub-sets of a given finite set
in this site.