SOLUTION: Given that A and B are sets such that n(A) = 8, n(B) = 12 and n(AnB) = 3, find n(AnB)

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Question 1195725: Given that A and B are sets such that n(A) = 8, n(B) = 12 and n(AnB) = 3, find n(AnB)
Answer by math_helper(2461) About Me  (Show Source):
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n(AnB) is 3, which is already given.  

You probably meant to ask 'find n(AuB)' (?), in which case n(AuB) = 8+12-3 = 17.

The number of elements in the intersection is subtracted once because it is counted twice (once in A and once in B).  See inclusion-exclusion principle.