Problem:
If set A has X elements, set B has Y elements and A U B (read A union B) has Z elements, how many elements does A ∩ B (read A intersection B) have?
Answer:
Since
A U B = A + B - (A ∩ B)
Then,
A ∩ B = A + B - (A U B)
elements in A ∩ B = X + Y - Z
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