\[\boxed{\text{5.378.}}\]
\[\textbf{а)}\ \frac{30}{36} = \frac{5 \cdot 6}{6 \cdot 6} = \frac{5}{6}\ \]
\[\textbf{б)}\ \frac{250}{200} = \frac{25 \cdot 10}{20 \cdot 10} = \frac{25}{20} = \frac{5 \cdot 5}{5 \cdot 4} =\]
\[= \frac{5}{4}\ \]
\[\textbf{в)}\ \frac{180}{270} = \frac{18 \cdot 10}{27 \cdot 10} = \frac{18}{27} = \frac{2 \cdot 9}{3 \cdot 9} =\]
\[= \frac{2}{3}\ \]
\[\textbf{г)}\ \frac{165}{330} = \frac{5 \cdot 33}{33 \cdot 10} = \frac{5}{10} = \frac{1}{2}\ \]