\[\boxed{\mathbf{28.}}\]
\[\textbf{а)}\ 30 + 60 = 90\]
\[48 - 2 = 46\]
\[55 - 40 = 15\]
\[4 + 70 = 74\]
\[63 - 3 = 60\]
\[27 - 10 = 17\]
\[49 + 1 = 50\]
\[85 - 30 = 55\]
\[50 + 17 = 67\]
\[\textbf{б)}\ 60 + 3 - 0 = 63\]
\[46 - (0 + 5) = 46 - 5 = 41\]
\[(47 + 0) - 30 = 47 - 30 = 17\]