\(x\in Bc\left(5;13\right)\)

\(\Rightarrow5=5\)   \(13=13\)

\(\Rightarrow5.13=65\)

\(\Rightarrow BC\left(5;13\right)=65\)

\(x\in Bc\left(5;13\right)\)     \(\left(13\le x\le78\right)\)

\(\Rightarrow5=5\)      \(13=13\)

\(\Rightarrow5.13=65\)

\(\Rightarrow BC\left(5;13\right)=65\)