Work Step by Step
Given: $n(A) = 15 , n(B) = 20$ and $n(A∩B)= 10$ Use the formula $n(A\cup B)= n(A) + n(B) - n(A\cap B)$ to obtain: $n(A\cup B)= n(A) + n(B) - n(A\cap B) \\=15+20-10 \\= 25$
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