01 If n(A)=52, n(B)=47, and n(A∪B)=79, what is n(A∩B)?
Answer and explanation
Correct answer: B. 20
Explanation: Use the two-set inclusion–exclusion identity n(A∪B)=n(A)+n(B)−n(A∩B). Rearranging gives n(A∩B)=n(A)+n(B)−n(A∪B). Substitution yields 52+47−79=99−79=20. The common elements must be subtracted because adding the two set totals counts them twice. Therefore, option B is the unique correct answer.