In a group of 40 students, 25 study biology and 20 study chemistry. If every student studies at least one of the two, how many study both?
Correct answer: A. 5
- A. 5
- B. 10
- C. 15
- D. 20
Explanation
By the inclusion exclusion principle, 25 plus 20 minus both equals 40, so both equals 5. The overlap is counted twice in the simple sum, which is why it must be subtracted once. Drawing two overlapping circles makes the relationship visible.
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