Negative Marks : -1 If wrong option is selected. If $n(A \cup B \cup C)+n(A \cup B \cup C)^{\prime}=50, n(A)=10, n(B)=15, n(C)=20, n(A \cap B)=5, n(B \cap C)=6, n(C \cap A)=10$ and $n(A \cap B \cap C)=5$, then $n(A \cup B \cup C)^{\prime}$ is equal to

Step 1 ：Using the principle of inclusion-exclusion: \(n(A cup B cup C) = n(A) + n(B) + n(C) - n(A cap B) - n(B cap C) - n(C cap A) + n(A cap B cap C)\)

Step 2 ：Plug in the given values and solve for \(n(A cup B cup C)^{prime}\): \(n(A cup B cup C) + n(A cup B cup C)^{prime} = 50\)

Step 3 ：\(n(A cup B cup C)^{prime} = 50 - n(A cup B cup C)\)

Step 4 ：\boxed{21}