\[\boxed{\mathbf{304.}}\]
\[\textbf{а)}\ 340 + 125 = 465\]
\[675 - 210 = 465\]
\[423 + 5 = 428\]
\[480 - 180 = 300\]
\[390 + 10 = 400\]
\[600 - 1 = 599\]
\[\textbf{б)}\ 712 + 85 - 6 = 797 - 6 =\]
\[= 791\]
\[895 - (36 + 36) = 895 - 72 =\]
\[= 823\]
\[542 + (73 - 46) = 542 + 27 =\]
\[= 569\]