\[0,4c = 0\ \Longrightarrow c = 0.\]
\[0,4c = 1\ \Longrightarrow c = \frac{1}{0,4} = 2,5.\]
\[0,4c = - 1\ \Longrightarrow c = - \frac{1}{0,4} = - 2,5.\]
\[0,4c = 10\ \Longrightarrow c = \frac{10}{0,4} = 25.\]