\[\boxed{\mathbf{206.}}\]
\[\textbf{а)}\ 423 + 300 = 723\]
\[423 + 30 = 453\]
\[423 - 20 = 413\]
\[576 - 20 = 556\]
\[576 - 200 = 376\]
\[576 - 500 = 76\]
\[\textbf{б)}\ 245 + 20 - 60 = 265 - 60 =\]
\[= 205\]
\[382 - 30 + 40 = 352 + 40 =\]
\[= 392\]
\[999 - (35 + 35) = 999 - 70 =\]
\[= 329\]