If n(A)=25 and n(B)=40 and n(A∪B)=50, find n(A Δ B).
Answer: C
The symmetric difference A Δ B is the set of elements in exactly one of the sets.
First, find the intersection: n(A∩B) = n(A)+n(B)-n(A∪B) = 25+40-50=15.
Now, n(A Δ B) = n(A∪B) - n(A∩B) = 50 - 15 = 35.
Alternatively: n(A only) = 25-15=10. n(B only) = 40-15=25. Total = 10+25=35.