If $n(A) = 10, n(B) = 6$ and $n(C) = 5$ for three disjoint sets A, B, C, then $n(A∪B∪C)$ equal ______.

21

Since A, B, C are disjoint sets. $∴n(A∪B∪C)=n(A)+n(B)+n(C)=10+6+5=21$