Three Differences and a Core

Difficulty: Medium · Topics: Discrete Mathematics, Sets, Venn Diagrams

About the numbers of elements of the sets A, B and C it is known that

n(Asetminus B)=n(Bsetminus C)=n(Csetminus A)=9

n(Acap Bcap C)=4

How many elements does the set Acup Bcup C have?