\[\boxed{\mathbf{2.}\mathbf{172}}\]
\[\textbf{а)}\ \frac{7}{20} + \frac{19}{50} = \frac{35}{100} + \frac{38}{100} = \frac{73}{100}\]
\[\frac{7}{20} + \frac{19}{50} = 0,35 + 0,38 = 0,73\]
\[\textbf{б)}\ \frac{4}{25} + \frac{3}{4} = \frac{16}{100} + \frac{75}{100} = \frac{91}{100}\]
\[\frac{4}{25} + \frac{3}{4} = 0,16 + 0,75 = 0,91\]
\[\textbf{в)}\ \frac{3}{5} - \frac{1}{25} = \frac{15}{25} - \frac{1}{25} = \frac{14}{25}\]
\[\frac{3}{5} - \frac{1}{25} = \frac{60}{100} - \frac{4}{100} =\]
\[= 0,6 - 0,04 = 0,56\]
\[\textbf{г)}\ \frac{4}{5} - \frac{99}{125} = \frac{100}{125} - \frac{99}{125} = \frac{1}{125}\]
\[\frac{4}{5} - \frac{99}{125} = \frac{80}{100} - \frac{792}{1000} =\]
\[= 0,8 - 0,792 = 0,008\]