\[5ab = 30\]
\[ab = 30\ :5 = 6\]
\[\Longrightarrow \ a = 3;\ \ \ \ b = 2.\]
\[5ab = - 10\]
\[ab = - 10\ :5 = - 2\]
\[\Longrightarrow a = - 1;\ \ b = 2.\]
\[5ab = 0\ \ \ \ \ \]
\[ab = 0\ :5 = 0\]
\[\Longrightarrow \ a = 5;\ \ \ \ b = 0.\]
\[5ab = 5\]
\[\ ab = 5\ :5 = 1\]
\[\Longrightarrow a = \frac{1}{2};\ \ \ \ \ b = 2.\]