\[\boxed{\mathbf{921\ (921)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\textbf{а)}\ a = 3;\ \ b = - \frac{2}{5}:\]
\[a - b = 3 - \left( - \frac{2}{5} \right) = 3 + \left( + \frac{2}{5} \right) = 3 + \frac{2}{5} = 3\frac{2}{5}.\]
\[\textbf{б)}\ a = - 0,11;\ \ b = - 0,3:\]
\[a - b = - 0,11 - ( - 0,3) = - 0,11 + ( + 0,3) = - 0,11 + 0,3 =\]
\[= 0,3 - 0,11 = 0,19.\]
\[\textbf{в)}\ a = - 10,2;\ \ b = 10,2:\]
\[a - b = - 10,2 - 10,2 = - (10,2 + 10,2) = - 20,4.\]
\[\textbf{г)}\ a = \frac{3}{8};\ \ b = - \frac{3}{8}:\]
\[a - b = \frac{3}{8} - \left( - \frac{3}{8} \right) = \frac{3}{8} + \left( + \frac{3}{8} \right) = \frac{3}{8} + \frac{3}{8} = \frac{6}{8} = \frac{3}{4}\text{.\ }\]