\[\boxed{\text{5.\ 50}}\]
\[\textbf{а)} - x \cdot ( - 8) = 8x \rightarrow 8\]
\[\textbf{б)}\ c \cdot ( - 5n) = - 5cn \rightarrow - 5\]
\[\textbf{в)}\ 4ab \cdot 7 = 28ab \rightarrow 28\]
\[\textbf{г)} - an \cdot ( - 8) = 8an \rightarrow 8\]
\[\textbf{д)}\ 4x \cdot ( - 5y) = - 20xy \rightarrow - 20\]
\[\textbf{е)} - 5a \cdot 0,2b = - ab \rightarrow - 1\]