When we are talking about sets, say A and B, I understand that IAI is the cardinality of set A and can be written as n(A) and the same for set B. So what does n(A-B) denote???

Question

When we are talking about sets, say A and B, I understand that IAI is the cardinality of set A and can be written as n(A) and the same for set B.  So what does n(A-B) denote???

zugzwang · Answer

A - B is the set of all elements in A but not in B, right?

anonymous · Answer

yes

anonymous · Answer

oh okay so please tell me if this is right:
Problem: If $$B \subseteq A, n(B)=n(A-B)$$ 
Prove if true.
My solution (according to what you guys said): 
FALSE because if the number of elements in B is say less than the number of elements in set A, then n(B)=n(A-B) is going to give the number of how many more elements are in A that are not in B. Right? For example if 15 elements are in both A and B but set A has 20 elements then n(B) is going to equal 5 by the formula given and that is false.