\(n(X\cup Y)=n(X)+n(Y)-n(X\cap Y)\)
If \(X\) and \(Y\) share no common elements, then \(n(X\cap Y)=0\), which means \(n(X\cup Y)=n(X)+n(Y)=47\)
On the other hand, if the smaller set is entirely contained within the larger set, i.e. \(Y\subseteq X\), then \(X\cap Y=Y\), and so \(n(X\cup Y)=n(X)+n(Y)-n(Y)=n(X)=28\).
