13. If \( n(A \cup B)=50, n(A \cap B)=10 \), and \( n(B)=20 \), find \( n(A) \).

Step 1 ：\( n(A \cup B) = n(A) + n(B) - n(A \cap B) \)

Step 2 ：\( 50 = n(A) + 20 - 10 \)

Step 3 ：\( n(A) = 50 - 20 + 10 \)