Discussion

If n(A Δ B) = 50, n(A) = 40, find n(B) if A and B are disjoint.

Answer: A

The symmetric difference A Δ B = (A ∪ B) - (A ∩ B).

If A and B are disjoint, their intersection is empty, so n(A ∩ B) = 0.

In this case, n(A Δ B) = n(A ∪ B) - 0 = n(A ∪ B).

Also for disjoint sets, n(A ∪ B) = n(A) + n(B).

Therefore, n(A Δ B) = n(A) + n(B).

50 = 40 + n(B) \(\Rightarrow\) n(B) = 10.

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