\[\boxed{\mathbf{2}\mathbf{.}\mathbf{115}}\]
\[\textbf{а)}\ 1836 + 640\ :8 =\]
\[= 1836 + 80 = 1916\]
\[\textbf{б)}\ 80 \cdot 11 - 42\ 558\ :519 =\]
\[= 880 - 82 = 798\]
\[\textbf{в)}\ 344\ :4 + 2456 =\]
\[= 86 + 2456 = 2542\]
\[\textbf{г)}\ 684 \cdot 245 - 675 \cdot 246 =\]
\[= 167\ 580 - 166\ 050 = 1530\]