Let $A, B, C$ be three finite sets. Given $n(A) = 20$, $n(B) = 25$, $n(C) = 30$. Also, $n(A \cap B) = 10$, $n(A \cap C) = 12$, and $n(B \cap C) = 15$. What is the minimum possible value for $n(A \cup B \cup C)$?