Chapter 12 - Counting and Probability - Section 12.1 Counting - 12.1 Assess Your Understanding - Page 867: 13

$n(A) = 40$

From the formula $n(A\cup B) = n(A) + n(B) - n(A \cap B)$ solve for $n(A)$ to obtain: $n(A\cup B) = n(A) + n(B) - n(A \cap B) \\n(A\cup B) +n(A \cap B) - n(B)=n(A) \\n(A)=n(A\cup B) +n(A \cap B) - n(B)$ Using the formula above gives: $n(A)= n(A\cup B) +n(A \cap B) - n(B) \\n(A)= 50+ 10 -20 \\n(A)= 40$