\[\boxed{\text{6.372.}}\]
\[\textbf{а)}\ (83,4 + 84,5)\ :2 =\]
\[= 167,9\ :2 = 83,95\]
\[\textbf{б)}\ (0,2 + 0,3 + 0,4)\ :3 =\]
\[= 0,9\ :3 = 0,3\ \]
\[\textbf{в)}\ (2,23 + 2,26 + 2,34 + 2,07)\ :4 =\]
\[= (4,3 + 4,6)\ :4 = 8,9\ :4 =\]
\[= 2,225\ \]
\[= (12,14 + 16,329 + 11,731) =\]
\[= (12,14 + 28,06)\ :6 =\]
\[= 40,2\ :6 = 6,7\]