\[\boxed{\text{5.336.}}\]
\[\textbf{а)}\ \frac{15c}{45c} = \frac{1}{3}\]
\[\textbf{б)}\ \frac{20m}{75m} = \frac{4}{15}\]
\[\textbf{в)}\ \frac{\text{mn}}{3m} = \frac{n}{3}\]
\[\textbf{г)}\ \frac{16ac}{8c} = \frac{2a}{1} = 2a\text{\ \ }\]