\[\boxed{\mathbf{990\ (990)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\textbf{а)}\ 0 - \frac{2}{9} = 0 + \left( - \frac{2}{9} \right) = - \frac{2}{9}\]
\[0 - ( - 3,4) = 0 + ( + 3,4) = 3,4\]
\[0 - 7,8 = 0 + ( - 7,8) = - 7,8\]
\[\textbf{б)}\frac{5}{7} - 1 = \frac{5}{7} - \frac{7}{7} = \frac{5 - 7}{7} = \frac{- 2}{7} = - \frac{2}{7}\]
\[0,15 - 2 = - (2 - 0,15) = - 1,85\]
\[1,6 - 3 = - (3 - 1,6) = - 1,4\]