\[\boxed{\mathbf{339\ (339).}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\boxed{\mathbf{339}\mathbf{.}}\]
\[1)\ x + \frac{7}{8} = 5\]
\[x = 5 - \frac{7}{8}\]
\[x = 4\frac{8}{8} - \frac{7}{8}\]
\[x = 4\frac{1}{8}.\]
\[2)\ x - 5\frac{8}{21} = 1\frac{17}{42}\]
\[x = 1\frac{17}{42} + 5\frac{8^{\backslash 2}}{21}\]
\[x = 1\frac{17}{42} + 5\frac{16}{42}\]
\[x = 6\frac{33}{42}\]
\[x = 6\frac{11}{14}.\]
\[3)\ 9\frac{1}{45} - x = 3\frac{1}{30}\]
\[x = 9\frac{1^{\backslash 2}}{45} - 3\frac{1^{\backslash 3}}{30}\]
\[x = 9\frac{2}{90} - 3\frac{3}{90}\]
\[x = 8\frac{92}{90} - 3\frac{3}{90}\]
\[x = 5\frac{89}{90}.\]