\[\boxed{\text{74\ (74).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ 5\frac{3}{4} - 2\frac{1}{7} + 1\frac{1}{4} - 4\frac{6}{7} =\]
\[= \left( 5\frac{3}{4} + 1\frac{1}{4} \right) - \left( 2\frac{1}{7} + 4\frac{6}{7} \right) =\]
\[= 7 - 7 = 0\]
\[\textbf{б)}\ 8\frac{2}{3} - 6\frac{3}{5} - 2\frac{2}{5} + 1\frac{7}{9} =\]
\[= \left( 8\frac{6}{9} + 1\frac{7}{9} \right) - \left( 6\frac{3}{5} + 2\frac{2}{5} \right) =\]
\[= 9\frac{13}{9} - 9 = \frac{13}{9} = 1\frac{4}{9}\]