\[\boxed{\text{5.371.}}\]
\[\textbf{а)}\ \frac{5}{6}\ и\ \frac{7}{12}\ \]
\[\frac{5}{6} = \frac{10}{12};\ \ \ \ \frac{7}{12}\text{.\ }\]
\[\textbf{б)}\ \frac{8}{15}\ и\ \frac{3}{5}\]
\[\frac{8}{15};\ \ \ \ \frac{3}{5} = \frac{9}{15}\text{.\ }\]
\[\textbf{в)}\ \frac{7}{16}\ и\ \frac{3}{8}\ \]
\[\frac{7}{16};\ \ \ \ \ \ \ \frac{3}{8} = \frac{6}{16}\text{.\ }\]
\[\textbf{г)}\ \frac{7}{10}\ и\ \frac{21}{40}\ \]
\[\frac{7}{10} = \frac{28}{40};\ \ \ \ \ \frac{21}{40}\text{.\ \ }\]