\[\boxed{\mathbf{343}\mathbf{.}}\]
\[1)\ 27\frac{1^{\backslash 2}}{4} - 25\frac{3}{8} = 27\frac{2}{8} - 25\frac{3}{8} =\]
\[= 26\frac{10}{8} - 25\frac{3}{8} = 1\frac{7}{8}\ \left( \frac{км}{ч} \right) -\]
\[скорость\ течения\ реки.\]
\[2)\ 27\frac{1^{\backslash 2}}{4} + 1\frac{7}{8} = 27\frac{2}{8} + 1\frac{7}{8} =\]
\[= 28\frac{9}{8} = 29\frac{1}{8}\ \left( \frac{км}{ч} \right) -\]
\[скорость\ катера\ по\ течению\ \]
\[реки.\]
\[Ответ:1\frac{7}{8}\ \frac{км}{ч};29\frac{1}{8}\ \frac{км}{ч}.\]