\[\boxed{\text{5.397.}}\]
\[\textbf{а)}\ \frac{7}{8} = \frac{35}{40};\ \ \frac{3}{4} = \frac{30}{40};\ \ \frac{3}{5} = \frac{24}{40};\]
\[\text{\ \ }\frac{17}{40}\text{.\ }\]
\[\frac{17}{40} < \frac{24}{40} < \frac{30}{40} < \frac{35}{40}\]
\[Расположим\ в\ порядке\ \]
\[убывания:\ \]
\[\frac{7}{8} > \frac{3}{4} > \frac{3}{5} > \frac{17}{40}\text{.\ }\]
\[\textbf{б)}\ \frac{11}{12} = \frac{22}{24};\ \ \frac{5}{24};\ \ \frac{5}{6} = \frac{20}{24};\]
\[\text{\ \ }\frac{2}{3} = \frac{16}{24}\text{.\ }\]
\[\frac{5}{24} < \frac{16}{24} < \frac{20}{24} < \frac{22}{24}\ \]
\[Расположим\ \ в\ порядке\ \]
\[убывания:\ \]
\[\frac{11}{12} > \frac{5}{6} > \frac{2}{3} > \frac{5}{24}\text{.\ \ }\]