Find the number of elements in the set A, if B has 36 elements and

Question

n ( A ∩ B ) = 18, n (A U B )= 55.

n(A) = __

Answer 1

n( A ∩ B ) = n(A) + n(B) - n(A U B )
18 = n(A) + 36 - 55

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