\[\boxed{\mathbf{2.}\mathbf{135}}\]
\[\textbf{а)}\frac{2}{4} = \frac{1}{2} = \frac{5}{10} = 0,5\]
\[\frac{2}{5} = \frac{4}{10} = 0,4\]
\[\frac{30}{50} = \frac{3}{5} = \frac{6}{10} = 0,6\]
\[\textbf{б)}\ \frac{1}{4} = \frac{25}{100} = 0,25\]
\[\frac{17}{20} = \frac{85}{100} = 0,85\]
\[\frac{9}{25} = \frac{36}{100} = 0,36\]
\[\frac{49}{50} = \frac{98}{100} = 0,98\]
\[\textbf{в)}\ \frac{3}{8} = \frac{3 \cdot 125}{8 \cdot 125} = \frac{375}{1000} = 0,375\]
\[\frac{13}{125} = \frac{13 \cdot 8}{125 \cdot 8} = \frac{104}{1000} = 0,104\]
\[\frac{161}{250} = \frac{161 \cdot 4}{250 \cdot 4} = \frac{644}{1000} = 0,644\]
\[\frac{173}{500} = \frac{346}{1000} = 0,346\]